The use of transmission electron microscopy tomography to correlate copper catalyst particle morphology with carbon fiber morphology

In this study carbon fibers of various morphologies have been synthesized by the decomposition of acetylene over copper supported catalysts (Cu(NO₃)₂/TiO₂, at 250 °C). In order to investigate the relationship between the morphologies of the copper catalyst particle and that of the carbon fiber extruded, tilting procedures (±60 A axis and ±30 B axis) under TEM (120 kV) were performed. In this way a 3-D (rather than a 2-D) image of the Cu crystallites were obtained which permitted for the 3-D shape of the Cu catalyst particle to be linked to the morphology of the extruded carbon fiber. Whereby copper particles that give distorted decahedra are apt to form helical fibers, trigonal bi-pyramidal particles give linear fibers. Various plate-like particles were also investigated, and it was observed that as the number of sides varied (3, 4, 5 or 6) there was a corresponding change in the carbon fiber helicity. Herein we report on our findings that reveal the geometric structures involved in controlling the shape of morphologically diverse carbon coils, illustrating the influential role catalyst particle shape plays in carbon fiber synthesis.     

Preparation of carbon micro-coils involving the decomposition of hydrocarbons using PACT (plasma and catalyst technology) reactor

Carbon micro-coils as well as carbon fibers with various morphologies were prepared by the decomposition of hydrocarbons, such as acetylene, methane, propane, ethylene, etc., at 770°C using a PACT (plasma and catalyst technology) reactor. The preparation conditions, growth mechanism and morphology of the carbon micro-coils were examined. The Ni electrode of the PACT reactor was used as the catalyst as well as a plasma source electrode. It was found that hydrocarbons, such as methane, propane and ethylene, decomposed under the plasma and catalyst atmosphere to form acetylene as the main decomposition product, and then this acetylene was further decomposed to form carbon micro-coils. Using a Ni powder catalyst dispersed on the substrate, the carbon micro-coils with a double helix structure, in which two pieces of carbon coils entwine each other in the same coiling direction, grew among the single straight carbon fibers and paired straight fibers. On the other hand, the carbon micro-coils with a single helix structure and wide coil pitch were obtained by the indirect decomposition of acetylene using the N₂ plasma formed by the PACT reactor.     

Synthesis of carbon nanocoils on substrates made of plant fibers

Carbon nanocoils (CNCs) with different shapes and coil diameters have been synthesized on three kinds of substrates made of plant fibers, i.e. tissue, cotton cloth and bamboo fiber cloth, using Fe₂ (SO₄)₃/SnCl₂ catalyst by a thermal chemical vapor deposition method. The average coil diameters of the CNCs on the tissue, cotton cloth and bamboo fiber cloth substrates are 560, 183, and 510 nm, respectively. It is found that the organization difference in the plant fiber substrates results in the difference in the aggregation states of catalyst particles on the fiber surfaces, which has a crucial effect on the morphology and production of the grown CNCs. The tight organization of the carbon fibers in the tissue and cotton cloth substrates can promote the catalyst aggregations to fabricate high yield CNCs. For the bamboo fiber cloth substrate, a relatively small number of catalyst particles are deposited on the surface and tend to be isolated, leading to the growth of a certain amount of the carbon nanofibers and carbon nanotubes. In addition, the catalyst adsorption ability of the bamboo fiber can be improved by coating calcium chloride particles to achieve high production of the regular CNCs.     

KOH-浸渍-还原法在炭纤维表面制备不同金属纳米催化剂涂层及其应用

为了催化炭纤维原位生长纳米炭纤维／纳米碳管，研究纳米炭纤维／纳米碳管在炭／炭复合材料中的应用，采用KOH-浸渍-还原法在炭纤维上制备纳米催化剂颗粒。首先用KOH处理炭纤维改变其形貌，然后将炭纤维分别在硝酸钴和硝酸镍催化剂前驱体溶液中浸渍，干燥，再用H2气还原制得催化剂颗粒，最后催化热解CO在炭纤维上原位生长纳米炭纤维／纳米碳管。结果表明：KOH处理能使炭纤维表面变得凹凸不平，有效的阻止了催化剂前驱体液体的流动，使涂层均匀；浸渍-还原法能获得粒径小、均匀、适合纳米炭纤维生长的金属颗粒；与Co纳米颗粒相比，Ni分散效果和催化效果更好。     

The effect of powder sintering on the palladium-catalyzed formation of carbon nanofibers from ethylene-oxygen mixtures

Carbon nanofiber growth on palladium particles from ethylene-oxygen mixtures was investigated with respect to thermal history. Electron microscopy, combined with focused ion beam cross-sectioning show particles sinter quickly, but can be stabilized by the addition of a short carbon deposition step at a temperature below the general reaction temperature. This step generates a thin layer of carbon on the catalyst which reduces sintering once the temperature is raised to the optimal reaction temperature. For example, high temperature (e.g. 500 °C) catalyst pre-treatment leads to catalyst particle sintering, and subsequent fiber growth produces large diameter fibers. In contrast, small diameter fibers form on catalyst particles pretreated at low temperature (ca. 350 °C), even if the fibers are grown at a temperature at which deposition rates are faster (e.g. 550 °C). These results led to the development of unique multiple temperature fiber growth protocols that produce smaller diameter fibers while improving the deposition rate.     

Helical carbon nanofibers with a symmetric growth mode

Helical carbon nanofibers with a symmetric growth mode were synthesized by the decomposition of acetylene with a copper catalyst. There were always only two helical fibers symmetrically grown over a single copper nanocrystal. The two helical fibers had opposite helical senses, but had identical cycle number, coil diameter, coil length, coil pitch, cross section, and fiber diameter. The irregular tips and helical reversals of the two helical fibers further revealed the symmetric growth mode. This mirror-symmetric growth mode was induced by the shape changes in copper nanocrystals during catalyzing the decomposition of acetylene. Upon contacting the initial copper nanocrystals with irregular shapes, acetylene began to decompose to form two straight fibers (the irregular tips). At the same time, shape changes in copper nanocrystals began. Once they changed from an irregular to a regular faceted shape, the two straight fibers ceased to grow and two regular helical nanofibers with opposite helical senses began to grow. If the regular faceted nanocrystals continue to change shapes during fiber growth, the two helical fibers possibly changed helical senses at the same time, resulting in helical reversals. The shape changes were caused by the changes in surface energy resulting from the acetylene-adsorption on the copper nanocrystals.     

Catalytic decomposition of light alkanes, alkenes and acetylene over Ni/SiO2

The decomposition of different hydrocarbons (CH₄, C₂H₆, C₂H₄, C₂H₂, C₃H₈, and C₃H₆) over Ni (5 wt.%)/SiO₂ catalysts was carried out. The initial rates of decomposition of the hydrocarbons, the kinetic curves of the decomposition and the kinetic curves of the hydrogenation of deposited carbon into methane depended on the types of hydrocarbons. In addition, the catalytic life of the Ni/SiO₂ catalyst was also dependent on the types of hydrocarbons, i.e. the life was longer according to the order, alkanes>alkenesacetylene.

The carbons deposited on the catalyst were characterized by SEM and Raman spectroscopy. The appearances of the deposited carbons were different among alkanes, alkenes, and acetylene, i.e. a zigzag fiber structure from methane, and a rolled fiber structure from alkenes and acetylene. From Raman spectra of the deposited carbons, it was found that the degree of graphitization of deposited carbon was higher in the order, alkanes>alkenes>acetylene. These results suggest that the mechanism of decomposition of hydrocarbons and the growth mechanism of carbon fibers on the catalyst were different among alkanes, alkenes and acetylene.     

不同原料基活性碳纤维的结构及吸附特征研究

采用Ｘ－射线衍射，ＳＥＭ和ＢＥＴ比表面分析仪，探讨了不同原料（聚丙烯脂，聚乙烯醇，粘胶，天然纤维和沥青碳纤维）基活性碳纤维的结构和吸附特征。实验表明，所制备的活性碳纤维都具有大致相同的乱层石墨结构，其微晶参数Ｌｃ，Ｌａ和ｄ值分别为１．０～１．２，３．９～５．０，０．３８～０．４０ｎｍ，微晶尺寸越小，活性碳纤维（ＡＣＦ）的比表面积越大。ＡＣＦｓ的表面形态都不相同且基本保持着原料纤维的形态，ＡＣＦｓ对有机物苯的吸附量与纤维的比表面积成正比，但对Ａｇ~＋的吸附则与比表面积关系不大，此外，吸附还原在ＡＣＦ上的Ａｇ粒形态与ＡＣＦ的表面形态有关，ＮＡＣＦ和ＡＡＣＦ上吸附的Ａｇ粒尺寸小于３００ｎｍ。     

Direct growth of carbon nanotubes on carbon fibers: Effect of the CVD parameters on the degradation of mechanical properties of carbon fibers

Grafting carbon nanotubes (CNTs) directly on carbon fibers represents a promising approach in order to strengthen the weak interface between carbon fibers and polymer matrix in carbon fiber reinforced polymer composites (CFRCs). We have carried out direct growth of CNTs on carbon fibers by using two different catalytic chemical vapor deposition (CVD) processes, namely the conventional CVD process based on catalytic thermal decomposition of ethylene and the oxidative dehydrogenation reaction between acetylene and carbon dioxide. The effect of various CVD growth parameters, such as temperature, catalyst composition and process gas mixture, was for the first time systematically studied for both processes and correlated with the mechanical properties of carbon fibers derived from single-fiber tensile tests. The growth temperature was found to be the most critical parameter in the presence of catalyst particles and reactive gasses for both processes. The oxidative dehydrogenation reaction enabled decreasing CNT growth temperature as low as 500 °C and succeeded to grow CNTs without degradation of carbon fiber's mechanical properties. The Weibull modulus even increased indicating partial healing of present defects during the CVD process. The new insights gained in this study open a way towards simple, highly reproducible and up-scalable process of grafting CNTs on carbon fibers without inducing any damages during the CVD process. This represents an important step towards CNT-reinforced CFRCs with higher damage resistance.     

Carbon nanotubes and helical carbon nanofibers grown by chemical vapour deposition on C60 fullerene supported Pd nanoparticles

Chemical vapour deposition (CVD) represents a cheap and versatile method to produce carbon nanostructures. Here we present how we by using a standard CVD setup together with Pd nano particles as a catalyst can produce helical fibers with very periodic pitch, helicity, and narrow diameter distribution. The C₆₀ supported Pd catalyst particles are produced by a wet chemistry process and applied to silicon substrates. By raising the growth temperature from 550 °C to 800 °C we can tune the growth products from helical carbon fibers to straight hollow carbon fibers and finally to carbon nanotubes at the highest temperatures. In the intermediate temperature region of 650 °C a mixture of all three components appears. At 550 °C the efficiency of the process is optimized by the amount of water during the growth. Different from most previous studies we can detect most of the catalyst particles embedded in the grown structures. In all fibers the catalyst particles are situated exactly in the middle of the fibers suggesting a two-directional growth. From the shape of the catalyst particles and by adopting a simple model we conclude that the fibers coil due to blocked carbon diffusion pathways on or through the catalyst particles.     

On the flame synthesis of carbon nanotubes grafted onto carbon fibers and the bonding force between them

A simple one-step method of the synthesis of carbon nanotubes (CNTs) grafted onto carbon fibers (CFs) was developed using catalysts formed in an ethanol flame, where the reducing atmosphere prevents the CFs from combustion and reduces the catalyst precursor to catalyst particles on the fibers. The growth process was studied and both the type and concentration of catalyst ions were found to affect CNT growth on CFs. The peeling force to separate the interfacial bonding between a CNT and a CF substrate was also measured by an instrumented tip of an atomic force microscope.     

Synthesis of carbon nanotubes on Ni/carbon-fiber catalysts under mild conditions

Methane, n-hexane, benzene, and cyclopentadiene were decomposed at a relatively mild temperature (773 K) over a Ni catalyst supported on either vapor grown carbon fibers (VGCF) or graphitized carbon fibers (GCF). Transmission electron microscopy showed that the morphology of the fibers changed according to hydrocarbon and particle size. Decomposition of methane and n-hexane produced fishbone-type fibers. The fibers from n-hexane sometimes showed intermittent hollow structures but the diameters of the fibers were widely distributed. Decomposition of benzene and cyclopentadiene mainly produced winding type carbon nanotubes of relatively uniform diameters (10-20 nm). Bidirectional fishbone-type fibers (fibers growing outward from a central catalyst particle) were also observed as a by-product. Small Ni particles (10-20 nm) with stretched tails were present on the tips of tubular fibers, some of which frequently changed growth direction. The varying tubular morphologies can be ascribed to liquid-like Ni particles resulting from the freezing point depression due to a fast dissolution of carbons during decomposition of benzene or cyclopentadiene. The formation of bidirectional fibers was also observed in the decomposition of n-hexane. Relatively large well-faceted Ni particles (diameter 50-110 nm) grew bidirectional fibers.     

High-yield growth and morphology control of aligned carbon nanotubes on ceramic fibers for multifunctional enhancement of structural composites

We present an in-depth study of CNT growth on commercially-available woven alumina fibers, and achieve uniform growth of dense aligned CNTs on commercially-available cloths up to 5 × 10 cm in area. By systematically varying the catalyst concentration, catalyst pre-treatment time, and sample position within the tube furnace, we isolate key factors governing CNT morphology on fiber surfaces and classify these morphologies as related to the processing conditions. Synthesis employs a low-cost salt-based catalyst solution and atmospheric pressure thermal CVD, which are highly attractive approaches for commercial-scale processing. The catalyst solution concentration determines the uniformity and density of catalyst on the fibers, H₂ exposure mediates formation of catalyst clusters, and thermal decomposition of the reactant mixture activates the catalyst particles to achieve uniform aligned growth. Under conditions for aligned CNT growth, uniform radially-aligned coatings are achieved with shorter CNT length, and these split into “mohawks” as the CNT length increases. Radially-aligned growth for 5 min adds a typical CNT mass fraction of 3.8% to the initial sample mass, and a uniform morphology exists throughout the weave. Composites prepared by standard layup techniques using these CNT “fuzzy” alumina fibers are attractive as integral armor layers having enhanced ballistic and impact performance, and serve as a model system for later implementation of this technology using carbon fibers.     

Experimental evaluation of the interfacial properties of carbon nanotube coated carbon fiber reinforced hybrid composites

A floating catalyst chemical vapor deposition (CVD) unit was utilized to grow CNT onto the surface of carbon fiber (CF). The surface morphology of the resultant fibers, CNT population density and alignment pattern were found to be depended on the CNT growth temperature, growth time, and atmospheric conditions within the CVD chamber. In contrast to the neat‐CF reinforced composites, improved interfacial shear strength (IFSS) between CF and matrix were obtained when the surface of CF was coated by CNT. Particularly, CF treatment condition for CNT‐coating with 700°C reaction temperature and 30 min reaction time has shown a considerable increase in IFSS approximately of 45% over that of the untreated fiber from which it was processed. The proper justification of fiber–matrix adhesion featured by composite interfacial properties was explained through IFSS. POLYM. COMPOS., 36:1941–1950, 2015. © 2014 Society of Plastics Engineers     

化学气相沉积法制备螺旋状纳米碳纤维研究

采用化学气相沉积方法在基体上生长螺旋状纳米碳纤维,以Ni为催化剂热解乙炔可制备出纯净、规则的单、双螺旋形碳纤维,单螺旋纤维的直径约为1—2μm,双螺旋的直径约为2—5wm;同时伴有少量的不规则螺旋形碳纤维的生成。拉曼光谱和X射线衍射分析证明螺旋形碳纤维是由小石墨微晶组成。根据机体法制备螺旋形的形貌、结构和生长过程,提出了螺旋形碳纤维的生长机制。     

Morphology of the growth tip of carbon microcoils/nanocoils

Single-helix twisted carbon nanocoils and single-helix spring-like carbon micro/nanocoils were prepared by the CVD process of the catalytic pyrolysis of acetylene at 700–800 °C over Fe-containing alloys in large scale, with a high purity and good reproducibility and with the coil yield of about 60%. The morphology and microstructure were examined. The catalyst grains on the growth tip of the nanocoils were observed closely. The results indicated that most of the twisted nanocoils grown by a two directional growth mode; that is, two twisting nanocoils grew out of a catalyst grain in opposite chirality and the fibers are grown in the herring-bone structure. Spring-like carbon coils were prepared with over sputtered Fe alloys on ceramics supporters, with a high purity and good reproducibility and with the coil yield of about 20%. All of the spring-like carbon coils are of one directional growth mode, and their coiling diameter and coil pitch are about the same size of several hundred nanometers. It is very common to observe spring-like nanocoils with laces, TEM investigation results showed that the spring-like nanocoils are of tubular coils.     

Acetylenes as probes in the Fischer–Tropsch Reaction

Long-chain acetylenic molecules (from C6 to C10) have been added as probe molecules to the synthesis gas feed in Fischer–Tropsch(FT) reactions on Co/Al₂O₃ at 0.7 Mpa and 150–220 °C. The acetylenic molecules initiated the reaction at temperatures as low as 150 °C. Use of a phenyl-substituted acetylene allowed the FT products initiated by the alkyne to be distinguished from base case FT synthesis. Terminal alkynes produced straight chain products while internal alkynes produced both terminal and internal products. Aldehydes and alcohols with one carbon more than the added acetylenic probe molecule were the only oxygenates formed, perhaps by a hydroformylation type of reaction.     

Helical nanocoiled and microcoiled carbon fibers as effective catalyst supports for electrooxidation of methanol

We have demonstrated a new synthesis of twist-shaped nanocoiled and herringbone-type double microcoiled carbon fibers via catalytic chemical vapor deposition of acetylene over NiCuMgAl-layered double hydroxides. The materials were characterized by power X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS) and N₂ adsorption-desorption experiments. For the first time, the use of electrodes modified with platinum particles supported on as-grown helical carbon fibers was studied for electrocatalytic oxidation of methanol. Especially, the electrode supported on herringbone-type double microcoiled carbon fiber showed much larger Pt active surface area than that supported on commercial carbon black. Furthermore, such electrocatalyst has exhibited about fourfold enhancement of activity and excellent anti-poisoning ability, which is believed to be attributed to the combined beneficial effects of novel microstructure and special composition of as-grown helical carbon fibers.     

Dry gel conversion synthesis of shape controlled MFI type zeolite materials

Acid treated silica fibers, coming either from asbestos cord waste or calcium aluminosilicate glass, were engaged as reactants to product MFI type zeolite fibers applying a dry gel conversion (DGC) type synthesis. The impact of amorphous silica fiber shape and synthesis conditions on morphology of the final product was investigated. Enhanced zeolite nucleation and restricted crystal growth are necessary to maintain the initial fiber morphology. Whereas a DGC type crystallization in hydroxide media results in the formation of zeolite fibers, the use of fluoride ions leads to the formation of large individual zeolite particles. Synthesis conditions have to be adapted to form zeolite particles with an average size smaller than the silica fiber diameter to allow a good conservation of the fiber morphology. Final zeolite fibers have textural properties comparable to a standard zeolite product and are promising for adsorption applications.     

Density control of carbon nanotubes and filaments films by wet etching of catalyst particles and effects on field emission properties

Carbon nanotubes and filaments (CNT&F) films with controlled density were grown by low pressure thermal chemical vapour deposition from acetylene on nickel nanoparticles. Density control was achieved by wet etching of the catalyst particles before carbon growth. Field emission measurements were carried out on several films with different CNT&F densities obtained with this method. Despite strong morphological changes, only slight differences in the field emission characteristics between the highest and the lowest density films were detected, suggesting that almost none of the CNT&F suppressed by the etching step took part to the field emission. However, a maximum field amplification factor was reached for the medium density film. Taking into account the field amplification factor distribution, a model is proposed to link the particles diameter distribution, the CNT&F film morphology and the field emission properties.