Ni催化剂在N2O／N2／NH3中退火对碳纳米管表面结构及场发射特性的影响

研究在N2O/N2/NH3氛围中对Ni催化剂进行退火处理,旨在探讨退火处理对所生成碳纳米管的表面结构及其发射特性的影响.从表面结构及表面元素分析结果发现:Ni催化剂在N2O/N2/NH3氛围中退火处理之后,Ni催化剂的颗粒大小及催化剂的化学成分发生改变,进而影响所合成的碳纳米管的表面结构及场发射特性.扫描电镜显示:经过N2O退火前处理后,催化金属薄膜在成核时较易形成均匀性的金属颗粒,且金属颗粒较小.比较经N2O/N2/NH3氛围退火处理之后所合成的碳纳米管结果,经过N2O前处理可以有效抑制非品质碳的成长,使所成长出的碳纳米管数量最多、场发射电流最大.原因主要是因为N2O对催化剂镍膜金属前处理过程中分解出的氮原子及氧原子会活化及氧化催化剂Ni金属,并使所形成的Ni金属颗粒较小且更为均匀,造成表面型态上的显著改变,有助于使合成的碳纳米管场发射电流变大.     

Diode-like properties of as-grown chemical vapor deposited single-walled carbon nanotubes

Current rectification property of as-grown single-walled carbon nanotubes (SWNTs) is investigated. The SWNTs are grown by chemical vapor deposition (CVD) process. The process allowed to grow long strands of SWNT bundles, which are then used to fabricate multiple arrays of switching devices with the channel length of 3, 5, 7 and 10 microm on a 15 mm x 15 mm SiO2 on Si substrate. Regardless of the channel length, a majority of the fabricated devices show current rectification characteristics, with high throughput of current (I) in the forward bias (V) giving the forward and reverse current ratio (Ifor/Irev) of approximately 10(6). Atomic force microscopic (AFM) analysis of the device structure and surface topology of SWNT suggest the observed rectification of current to possibly result from (a) cross-tube junctions, (b) a mixture of metallic and semiconducting tubes in the SWNT bundles, and/or (c) chirality change along a single tube. The exact mechanism underlying the observed rectification could not be conclusively established. However, the analyses of the experimental results strongly suggest the observed rectification to result from Schottky-type diode properties of SWNTs with mixed chirality along the tube.     

热化学气相沉积法在硅纳米丝上合成碳纳米管

利用热化学气相沉积法在负载不同厚度催化剂的硅纳米丝(SiNW)表面生长碳纳米管(CNTs),探讨了生长条件对所合成SiNW-CNT的结构和场发射特性的影响.这种类似树状的三维结构具有较高碳纳米管表面密度及降低的电场筛除效应等潜在优势.使用拉曼光谱( Raman)、电子显微镜(SEM)、透射电子显微镜(TEM)、能量扩散分光仪(EDS)分析了碳纳米管的结构性质,并在高真空下施加电场测得碳纳米管的场发射特性.结果表明:随硅纳米丝上负载催化剂镍膜厚度的变化,所合成碳纳米管的表面特性、结晶结构及功函数改变,导致电子发射难易程度的改变,进一步影响碳纳米管的场发射特性.     

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.     

Effects of hydrogen on the formation of aligned carbon nanotubes by chemical vapor deposition

Well-aligned carbon nanotubes with controllable properties were grown on porous silicon substrates by thermal chemical vapor deposition. The morphologies of the carbon nanotubes were varied with the introduction of H2 during the catalyst activation and/or carbon nanotube growth processes. It was found that H2 promotes the growth of carbon nanotubes while preventing the formation of spherical amorphous carbon particles. Without the introduction of H2 during the C2H2 thermal decomposition, aligned carbon nanotubes mixed with spherical carbon particles were formed on the substrate. However, with the introduction of H2, pure carbon nanotubes were synthesized. These nanotubes also had uniform diameters of 10-20 nm, which is much smaller than nanotubes synthesized without H2. The average growth rate of nanotubes was also affected by the introduction of hydrogen into the reaction chamber during nanotube growth. With the addition of hydrogen, the average growth rate changed from 78 nm/s to 145 nm/s. A possible growth mechanism, including the effect of a high ratio of H2 to C2H2, is suggested for the growth of these well-aligned carbon nanotubes with uniform diameters.     

乙炔热裂解沉积对PAN基炭纤维性能的影响

利用有机溶剂去除PAN基炭纤维表面的集束剂与染剂.然后通过乙炔热裂解沉积对其进行表面改性,以期获得兼具高机械强度和优良导电性的高性能PAN基炭纤维.采用SEM、AFM、XRD、Raman等方法对PAN基炭纤维在改性前后的微观结构、结晶性、抗拉强度、弹性模量、导电性等进行了分析.研究结果表明采用化学气相沉积法可以提高或者明显改善石墨化处理后的PAN基炭纤维的力学性能(抗拉强度为2GPa,弹性模量为270GPa)和导电性(5×10-4Ω·cm).     

改进化学气相沉积法在炭纤维表面生长碳纳米管(英文)

采用一种改进的化学气相沉积法在炭纤维表面制备碳纳米管。为了提高炭纤维表面的润湿性能,炭纤维在浸渍之前先在CVD设备中在真空下973 K的高温处理,然后在硝酸和浓硫酸体积比为3∶1的混合酸中酸处理30 min。而改进的化学气相沉积法关键在于让催化剂的还原步骤和碳纳米管的生长步骤同时进行。这样通过减小过渡金属元素与炭纤维之间的接触时间从而降低了它们之间的相互扩散,在确保了炭纤维本身的力学性能下降程度明显小于用普通化学气相法制备的情况下生长出长且茂密的碳纳米管阵列。另外,经过对工艺参数的优化发现当用乙醇作溶剂,Fe(NO3)3.9H2O溶度为100 mmol/L,氢气和碳源气体比值为4/1,而生长时间为30 min时得到最好的碳纳米管阵列。     

Growth of long aligned carbon nanotubes on amorphous hydrogenated silicon nitride by thermal chemical vapor deposition

Vertically aligned long carbon nanotubes in the range of 80-100 µm have been synthesized on amorphous hydrogenated silicon nitride (a-SiN_x:H) coated silicon substrate by thermal chemical vapor deposition of ferrocene and xylene. It is observed that high temperature annealing in oxygen ambient results in formation of crystalline silicon dioxide in the matrix of amorphous silicon nitride due to out diffusion of hydrogen. It is suggested that active sites created on silicon dioxide and a-SiN_x:H clusters provide mechanical support for the alignment of long carbon nanotubes. It is proposed that a thin layer of a-SiN_x:H prevents silicide formation between the catalyst (Fe) and silicon thus lengthening the catalyst life.     

Effect of nitrogen and hydrogen on the growth of multiwall carbon nanotubes on flexible carbon cloth using thermal chemical vapor deposition

In this study, the effect of various mixture fluxes of nitrogen (N₂) and hydrogen (H₂) on carbon nanotube (CNT) synthesis grown on flexible carbon cloth using thermal chemical vapor deposition (thermal CVD) with ethylene (C₂H₄) as the carbon source and nickel (Ni) as the catalyst was investigated. Field emission scanning electron microscopy (FE-SEM) was utilized to study the morphology of CNTs on flexible carbon cloths with various N₂ and H₂ inlet flow rates. The results indicate that average diameter of MWCNTs decreases with increasing H₂ and N₂ flow rates; however, the density of CNTs increases first and then decreases with increasing H₂ and N₂ flow rates. On the other hand, in our field emission experiments, the result indicates that the field emission is strongly dependent on the density and geometry of MWCNTs. In addition, we also found that the contact electrical conductance measurement is an easy method to predict the field emission characteristics of MWCNTs.     

Effects of methane partial pressure on synthesis of single-walled carbon nanotubes by chemical vapor deposition

A series of experiments have been done to investigate the role of methane partial pressure in the synthesis of carbon nanotubes by catalyst chemical vapor deposition. It is supposed that there is a critical methane partial pressure (0.4 atm) for single-walled carbon nanotube (SWNT) synthesis at 850°C. When the methane partial pressure is higher than the critical value, the whole rate is decided by the carbon diffusion rate on the catalyst surface or in the bulk, contrariwise, the synthesis rate is proportional to the methane partial pressure.     

Effects of substrate size on properties of carbon coatings on glass cylinder substrates prepared by thermal chemical vapor deposition

The effect of the substrate size on the properties of carbon coatings that are deposited on the glass cylinder substrates by thermal chemical vapor deposition is investigated. Experimental results show that the deposition rate of carbon coatings decreases as the diameter of the glass cylinder increases, because the residence time of the precursor gas in the deposition zone decreases and the deposition area of the substrate increases. Experimental results also reveal that the surface roughness and electrical resistivity of carbon coatings decrease as the diameter of the glass cylinder increases, while the degree of ordering and crystallite size of the carbon coatings increase.     

Stacking nature of the catalytic chemical vapor deposition-derived double-walled carbon nanotubes

Here we described the synthesis of highly pure double walled carbon nanotube (DWNT) through a right combination of a catalytic chemical vapor deposition method and the two-step purification and evaluated their stacking infidelity when compared to multi-walled carbon nanotubes (MWNTs). Easy fabrication of thin and flexible, but mechanically tough DWNT-buckypaper was due to the long and large-sized bundled DWNT (up to 50 nm), where DWNTs with diameter below 2 nm were packed in hexagonal array. Through detailed transmission electron microscope, X-ray and Raman studies, we confirmed that the intershell spacing of our DWNT sample was ca. 0.36 nm, which was believed to strongly affect negative and small magnetoresistance absolute value of -0.09 at 77 K and 1 T.     

Correlation between chemical vapor deposited diamond and carbon fibers substrates

Diamond film formation has been studied on carbon felts (CF) substrates produced from polyacrylonitrile precursor, at different heat treatment temperatures (HTT). Scanning electron microscopy images have revealed a polycrystalline and preferential (111) diamond film covering the whole CF surface, even for deeper planes. The average grain size increased from 3.0 up to 6.0 μm for films grown on CF treated between 1000 and 2000 °C. This behavior may be attributed to different contributions associated to the facility to extract carbons atoms from CF substrate. For CF treated at lower HTT, higher carbon atoms amount will promote higher nucleation density on diamond films and consequently a smaller grain size. Raman spectroscopy indicated good quality diamond films and the lower amount of graphitic phase was observed for diamond grown on CF obtained at 2000 °C HTT. The microstructural properties of the CF were obtained by X-ray diffraction (XRD) and show a strong dependence with HTT.     

Reactor length-scale modeling of chemical vapor deposition of carbon nanotubes

Chemical Vapor Deposition (CVD) of carbon nanotubes from a gas mixture consisting of methane (carbon precursor) and hydrogen (a carrier gas) in the presence of cobalt, nickel or iron catalytic particles in a cylindrical reactor is modeled at the reactor length-scale by solving a continuum-based coupled boundary-layer laminar-flow hydrodynamics, heat-transfer, gas-phase chemistry and surface chemistry problem. The model allows determination of the gas-phase fields for temperature, velocity, and various species as well as the surface-species coverages and the carbon deposition rate. Various available experimental and theoretical assessments are used to construct the necessary database for gas-phase and surface chemistry and gas-phase transport parameters. A reasonably good agreement is found between the model predicted and the experimentally measured carbon nanotubes deposition rates over a relatively large range of processing conditions.     

Fabrication and characteristics of field emitter using carbon nanotubes directly grown by thermal chemical vapor deposition

We experimentally present the effects of vertical alignment and density of carbon nanotubes on the emission current level. For practical display application, we have fabricated the triode type emitter using directly grown nanotubes as emission tip, and characterized their basic field emission properties. The triode type emitter exhibited a turn-on voltage of 37 V and an anode current density of 1.7 μA with gate voltage at 47 V. The vertical alignment of nanotubes does not play a key role in improving the emission properties in triode type nanotubes emitter.     

Microstructural and optical properties of nanocrystalline ZnO deposited onto vertically aligned carbon nanotubes by physical vapor deposition

Nanocrystalline ZnO films with thicknesses of 5 nm, 10 nm, 20 nm, and 50 nm were deposited via magnetron sputtering onto the surface of vertically aligned multi-walled carbon nanotubes (MWCNTs). The ZnO/CNTs heterostructures were characterized by scanning electron microscopy, high resolution transmission electron microscopy, and X-ray diffraction studies. No structural degradation of the CNTs was observed and photoluminescence (PL) measurements of the nanostructured ZnO layers show that the optical properties of these films are typical of ZnO deposited at low temperatures. The results indicate that magnetron sputtering is a viable technique for growing heterostructures and depositing functional layers onto CNTs.     

化学气相沉积法制备多壁碳纳米管

以带程序升温装置的管式电阻炉为实验装置,采用化学气相沉积法,在一定的工艺条件下裂解二茂铁与双鸭山精煤的混合物制备出多壁碳纳米管.采用透射电镜、Raman光谱以及X射线衍射技术对碳纳米管产物进行表征,同时研究了碳纳米管的生长机理.     

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.     

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.     

Structure of multiwalled carbon nanotubes grown by chemical vapor deposition

Carbon nanotubes have been grown by chemical vapor deposition at 650°C in an argon atmosphere using a butane-propane mixture and a nickel catalyst and have been characterized by scanning and transmission electron microscopy and Raman spectroscopy. The results indicate that the multiwalled nanotubes have an imperfect graphite-like structure with a conical supramolecular configuration. A phenomenological technique is proposed for statistical analysis of the state of carbon nanotubes in measurements of the intensity of the defect zone D in their Raman spectra.