反应时间对CVD法制备多壁碳纳米管产率和形貌的影响

以甲烷为碳源气,以氢气为载气和还原气,以自制纳米NiO/SiO2气凝胶为催化剂,探讨了气相化学沉积法制备碳纳米管工艺过程中,反应时间对产物产率和形貌的影响.实验结果表明,随反应时间的延长,碳纳米管的产率不断提高,一定时间后增长趋于平缓,TEM图象表明碳纳米管的长径比也随之增加.     

反应时间对VCD法制备多壁碳纳米管产率和形貌的影响

以甲烷为碳源气，以氢气为载气和还原气，以自制纳米NiO/SiO2气凝胶为催化剂，探讨了气相化学沉积法制备碳纳米管工艺过程中，反应时间对产物产率和形貌的影响。实验结果表明，随反应时间的延长，碳纳米管的产率不断提高，一定时间后增长趋于平缓，TEM图象表明碳纳米管的长径比也随之增加。     

LaAl1/3Fe2/3O3催化剂上正己烷裂解制备碳纳米管研究

用硝酸镧、硝酸铝与硝酸铁作原料采用柠檬酸法制备了纳米铁铝酸镧催化剂,以正己烷作碳源催化裂解制备了高产率管径均匀的碳纳米管.实验考察了裂解温度、裂解时间和氢气气氛对碳纳米管形貌、产率、直径分布等的影响,采用透射电镜、扫描电镜对碳纳米管进行了表征.结果表明氢气气氛对正己烷催化裂解制备碳纳米管有明显影响,较好的裂解条件是温度为1 023 K,时间为45min,氢气含量为30%;此时碳纳米管管径较小、分布比较均匀.     

工艺参数对浮游催化法制备碳纳米管的影响

采用浮游催化法喷雾进料技术制备碳纳米管。研究表明，工艺参数如催化剂含量、硫添加量和氢气流量对产物收率、形貌和微观结构有显著影响；低催化剂含量、合适的硫添加量和高氢气流量有利于较细直径碳纳爿管的生成。通过优化工艺参数可以制备出平均直径为35m的均匀纯净的碳纳米管。     

化学气相沉积法低温制备氢气和多壁碳纳米管

采用低温化学气相沉积法裂解乙醇协同制备氢气和多璧碳纳米管,乙醇作为碳源,Ni/C用作催化剂。考察反应温度和Ni/C比例对氢气的产率和碳纳米管品质的影响。多壁碳纳米管结构与组成通过扫描电镜、投射电镜和X射线粉末衍射进行表征。结果表明:Ni/C催化剂最佳Ni担载量为8%,最佳的反应温度为500℃,最佳条件下氢气的产率为79%,碳纳米管的品质最佳。     

乙醇裂解协同制备氢气和碳纳米管

以乙醇为碳源,采用浸渍法制备的担载量为Fe(5%)/C催化剂,利用化学气相沉积法协同制备碳纳米管和氢气,分析了裂解温度(500⁸00℃)对于产生氢气产率和碳纳米管品质的影响。对于Fe(5%)/C催化裂解乙醇,最佳的反应温度为600℃,碳管的品质较好,氢气的产率最高为75%,生成的碳管为多壁碳纳米管。     

加氢催化剂活性评价反应工艺条件研究

以Pd/Al2O3催化剂为例,选取了苯乙烯加氢制备乙苯的体系,探讨了一种在常压、低温、间歇反应评价催化剂活性的方法,利用单因素实验,考察了搅拌速度、反应温度、反应时间、氢气流量、溶剂用量对评价反应的影响;在单因素实验的基础上,运用响应面分析法中的Box-Benhnken模块进行三因素三水平的实验设计,以苯乙烯转化率作为响应值,进行响应面分析、优化和实验验证,得出最佳工艺参数为反应温度67.24℃,反应时间2.95h,氢气流量150ml/min,产率可达到100.00%。     

常压高效合成双（烷基聚氧乙烯基）仲胺

针对仲胺基聚醚合成存在产率低、反应条件苛刻等问题,以端伯胺基C12～14烷基聚乙二醇醚(A/C12～14E5)为原料,通过伯胺在常压条件的分子间脱氨耦合,高效地合成了双(C12～14烷基聚氧乙烯基)仲胺(S/C12～14E5).探究了耦合温度、催化剂用量、氢气流量及反应时间对S/C12～14E5选择性的影响.结果表明,当反应温度为200℃,催化剂用量为原料总质量的2.5％,氢气流量为200 mL/min,反应时间为5 h时,仲胺含量(化学法)达到97.3％.在此基础上,进行了底物拓展、放大实验,并提出了可能的反应机理.     

碳包铁颗粒和放射状碳纳米管微观结构的研究

以苯和甲苯为碳源，二茂铁为催化剂前驱体，含硫化合物为助催化剂，采用竖式炉流动催化法，通过减小载人的氢气量以改变催化剂颗粒的状态及反应条件，获得了碳包铁颗粒以及放射状碳纳米管，运用TEM和HRTEM对其形貌和结构进行了分析，并初步探讨了其生长机理。结果表明，在碳源、催化剂和炉温分布相同的条件下，氢气量为5400mL／min时形成直线型和弯曲型两种不同形态的碳纳米管，后者管径大于前者。氢气量为2000mL／min时，产物90％以上为碳包铁颗粒，其平均直径约为530nm，其中还有少量放射状碳纳米管，其外径为45—50nm，内径为3—5nm，管径较为均匀。     

碳纳米管改性对Au/CeO2催化剂乙醇部分氧化制氢的影响

采用沉积沉淀法制备了一系列碳纳米管改性的Au/CeO₂催化剂,以乙醇部分氧化制氢为探针反应,研究了碳纳米管对Au/CeO₂催化剂乙醇部分氧化性能的影响,并运用XRD、TPR、BET等方法对催化剂进行了表征。结果表明,碳纳米管的添加提高了Au/CeO₂催化剂的比表面积、孔容和吸氧量,催化剂的氢气选择性先随碳纳米管添加量的增加而大幅增加,碳纳米管的添加量达6%～10%时,氢气选择性达到43%。进一步提高碳纳米管的含量,氢气选择性增加幅度不大。碳纳米管的添加可以有效抑制副产物CO的产生。     

Scaled-up self-assembly of carbon nanotubes inside long stainless steel tubing

The self-assembly of carbon nanotubes (CNTs) on the inside wall of a relatively long stainless steel tubing for applications such as separations and chromatography, is reported in this paper. The CNTs were deposited by the chemical vapor deposition (CVD) using ethylene as the carbon source and the iron nanostructures in the stainless steel as the catalyst. The coating consisted of a layer of CNTs aligned perpendicular to the circumference of the tubes, often with an overcoat of disordered carbonaceous material, which could be selectively oxidized by exposing the CNT layer below to pure O₂ at 375 °C. Variation in uniformity in terms of the thickness and morphology of the deposited film and surface coverage were studied along the length of a tube by scanning electron microscopy (SEM). The effects of process conditions, such as flow rate and deposition time on the coating thickness, were studied. The catalytic effect of the iron nanostructures depended on surface conditioning of the tubing. It was found that the pretreatment temperature influenced the quality of the nanotube coating. The morphology of the CNT deposit supported the base-growth scheme and VLS (vapor-liquid-solid) growth mechanisms of CNTs.     

Experimental characterization of the role of hydrogen in CVD synthesis of MWCNTs

An experimental study was conducted to examine the role of hydrogen in the chemical vapor deposition (CVD) synthesis of multiwalled carbon nanotubes (MWCNTs) in a flow tube reactor using xylene as a carbon source and ferrocene as a catalyst. Ferrocene was introduced into the reactor by two methods. In a single step method, the catalyst was dissolved in xylene and the mixture was introduced using a syringe pump. A two step method was also used where the ferrocene powder was placed in the preheated zone for a certain time to deposit iron catalyst particles on the reactor wall prior to introducing the pure xylene into the reactor. CVD synthesis of carbon products was performed as a function of hydrogen input under constant flow conditions using both methods. SEM and TEM images of the carbon products were examined. The results revealed a competition between the formations of the different carbon products (soot, carbon fibers and CNTs) that altered by the addition of hydrogen. The role of hydrogen is suggested to reduce the rate of carbon production by dehydrogenation so that the more ordered and thermodynamically stable MWCNTs can be produced rather than less ordered and thermodynamically stable soot and carbon fibers.     

A time‐dependent multiphysics,multiphase modeling framework for carbon nanotube synthesis using chemical vapor deposition

A time‐dependent multiphysics, multiphase model is proposed and fully developed here to describe carbon nanotubes (CNTs) fabrication using chemical vapor deposition (CVD). The fully integrated model accounts for chemical reaction as well as fluid, heat, and mass transport phenomena. The feed components for the CVD process are methane (CH₄), as the primary carbon source, and hydrogen (H₂). Numerous simulations are performed for a wide range of fabrication temperatures (973.15–1273.15 K) as well as different CH₄ (500–1000 sccm) and H₂ (250–750 sccm) flow rates. The effect of temperature, total flow rate, and feed mixture ratio on CNTs growth rate as well as the effect of amorphous carbon formation on the final product are calculated and compared with experimental results. The outcomes from this study provide a fundamental understanding and basis for the design of an efficient CNT fabrication process that is capable of producing a high yield of CNTs, with a minimum amount of amorphous carbon. © 2009 American Institute of Chemical Engineers AIChE J, 2009     

New continuous gas-phase synthesis of high purity carbon nanotubes by a thermal plasma jet

We have developed a new gas-phase synthesis technique to produce carbon nanotubes (CNTs) with a continuous process and at high temperature, by using a thermal plasma jet. A thermal plasma jet was generated by applying a direct current of 100-300 A, using Ar as the plasma gas with a flow rate of ∼6 ksccm. The temperature of the thermal plasma jet was very high (∼10⁴ K) and the velocity was very fast (∼100 m/s). Fe(CO)₅ and CO were used as a catalyst precursor and carbon source, respectively. The yield of CNTs was dramatically increased by attaching a helical extension reactor at the end of the plasma nozzle. High purity (∼80%) CNTs were produced with a continuous process by using a thermal plasma jet with helical extension reactor equipment. The number of CNT walls produced was critically affected by the hydrogen gas injected as an auxiliary plasma gas. Without hydrogen gas, single-walled carbon nanotubes whose diameter was about 1 nm were mostly produced while with hydrogen gas double-walled carbon nanotubes (about 4 nm in diameter) were predominantly produced, with small amount of 3- and 4-walled carbon nanotubes.     

Thermal conductivity of natural rubber nanocomposites with hybrid fillers

Natural rubber nanocomposites filled with hybrid fillers of multi-walled carbon nanotubes(CNTs) and carbon black(CB) were prepared. CNTs were ultrasonically modified in mixture of hydrogen peroxide(H_2O_2) and distilled water(H_2O). The functional groups on the surface of CNTs, changes in nanotube structure and morphology were characterized by Fourier transform infrared spectroscopy(FT-IR), Raman Spectroscopy, and transmission electron microscopy(TEM). It shows that hydroxyl(OH·) is successfully introduced. The surface defects of modified CNTs were obviously higher than those of original CNTs, and the degree of agglomeration was greatly reduced. Thermal conductivity of the composites was tested by protection heat flow meter method. Compared with unmodified CNTs/CB filling system, the thermal conductivity of hybrid composites is improved by an average of 5.8% with 1.5 phr(phr is parts per hundred rubber) of hydroxyl CNTs and 40 phr of CB filled. A three-dimensional heat conduction network composed of hydroxyl CNTs and CB, as observed by TEM, contributes to the good properties. Thermal conductivity of the hybrid composites increases as temperature rises. The mechanical properties of hybrid composites are also good with hydroxyl CNTs filled nanocomposites; the tensile strength, 100% and 300% tensile stress are improved by 10.1%, 22.4% and 26.2% respectively.     

Effects of bimetallic catalyst composition and growth parameters on the growth density and diameter of carbon nanotubes

Multi-wall carbon nanotubes (MWNTs) were synthesized by catalytic decomposition of acetylene over Fe, Ni and Fe-Ni bimetallic catalysts supported on alumina under various controlled conditions. The growth density and diameter of CNTs were markedly dependent on the activation time of catalysts in H₂ atmosphere, reaction time, reaction temperature, flow rate of acetylene, and catalyst composition. Bimetallic catalysts were apt to produce narrower diameter of CNTs than single metal catalysts. For the growth of CNTs at 600 ‡C under 10/100 seem flow of C₂H₂/H₂ mixture, the narrowest diameter about 20 nm was observed at the reaction time of 1 h for 20Fe : 20Ni : 60Al₂O₃ catalyst, but at that of 1.5 h for 10Fe : 30Ni : 60Al₂O₃ catalyst. It was considered that the diameter and density of CNTs decreased with the increase of the growth time mainly due to hydrogen etching. The growth of CNTs followed the tip growth mode.     

连续流动状态下纳米碳管负载镍催化剂催化苯乙炔选择加氢反应

采用浸渍法,以纳米碳管（CNT）为载体,制备了Ni负载量为3.91%（质量分数）的Ni/CNTs催化剂。利用X射线衍射（XRD）、拉曼光谱、电感耦合等离子体发射光谱（ICP-OES）以及透射电镜（TEM）等技术对Ni/CNTs催化剂进行了表征。在连续流动状态下通过一系列实验考察了温度、压力、氢气量以及流速对Ni/CNTs催化苯乙炔选择加氢性能的影响,利用高效液相色谱仪HPLC对产品进行定性和定量分析。实验结果表明,催化剂Ni/CNTs对苯乙炔选择加氢反应具有一定的催化活性;反应的最佳温度为20℃;压力对产品收率影响显著,微正压对反应有利;最佳氢气量为24mL/min;产品收率随着反应液流速的增加而增大。     

CVD growth of secondary carbon nanotubes and its effect on field electron emission

Secondary carbon nanotubes (CNTs) were grown on primary ones by simply changing the methane concentration. No additional catalyst was used throughout the whole deposition process. The CNT growth was carried out using hot filament chemical vapor deposition in a gas mixture of methane and hydrogen. The structure and surface morphology of the deposited CNTs were studied and the field emission properties of the CNTs were tested. It was found that synthesizing primary CNTs at extremely low methane concentration is the key for the secondary growth without additional catalyst. The CNT samples grown with secondary nanotubes exhibited improved field emission properties.     

Effect of carbon nanotube purification on the electrical and mechanical properties of poly(ethylene terephthalate) composites with carbon nanotubes in low concentration

Surface properties of carbon nanotubes (CNTs) were altered by purification with nitric acid, sulfuric acid, ammonium hydroxide, and hydrogen peroxide. As‐received and purified CNT‐based conductive poly (ethylene terephthalate) composites were prepared with a twin‐screw extruder. The effects of CNT purification on the surface properties of the CNTs and on the morphology and electrical and mechanical properties of CNT‐based composites were investigated. Surface energy measurements showed that the acidic component of the surface energies of the CNTs increased after purification. According to Fourier transform infrared (FTIR) spectroscopy, the purification resulted in the formation of oxygen‐containing functional groups on the surfaces of the CNTs. Electron spectroscopy for chemical analysis results indicate the removal of the metallic catalyst residues and an increase in the oxygen content of the CNT surfaces as a result of the purification procedure. X‐ray diffraction analyses revealed a change in the crystalline structure of the CNTs after purification. All of the composites prepared with the purified CNTs had higher electrical resistivities and tensile and impact strength values than the composites based on the as‐received CNTs because of the functional groups and defect sites formed on the surfaces of the CNTs during purification. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Study of carbon nanotubes synthesis by spray pyrolysis and model of growth

Multi-walled carbon nanotubes (MWCNTs) were grown inside of quartz tubing by spray pyrolysis of ferrocene/benzene under argon flow. Carbon nanotubes (CNTs) with length of 200 μm were produced with reaction time of 10 min. The diameter of CNTs was influenced by the size of droplets formed in the nebulizer and the length was greatly influenced by ferrocene concentration and argon gas flow. It was found that temperature is a critical variable to produce CNTs at the experimental conditions used in this work. It was also found that CNTs only grew if ferrocene is added to gas flow, even if CNTs are previously seeded and formed on substrate, benzene cannot produce the CNTs without ferrocene. A model of CNTs formation and growth is proposed for spray pyrolysis of ferrocene/benzene, this mechanism consist of the formation of carbon/Fe nanoparticles during pyrolysis in the gas phase, these nanoparticles reach the walls of substrate, and the nanoparticles attach to substrate surface or to the nanotubes. Under proper conditions the displacement of Fe inside the graphitic structure induces the alignment of carbon walls, straightening this way the nanotubes.