Oxidative stabilization of PAN/SWNT composite fiber

PAN/SWNT composite fibers have been spun with 0, 5, and 10 wt% single wall carbon nanotubes (SWNTs). Tensile fracture surfaces of polyacrylonitrile (PAN) fibers exhibited extensive fibrillation, while for PAN/SWNT composite fibers, tendency to fibrillate decreased with increasing SWNT content. The reinforcing effect of SWNTs on the oxidized polyacrylonitrile (PAN) fiber has been studied. At 10 wt% SWNTs, breaking strength, modulus, and strain to failure of the oxidized composite fiber increased by 100%, 160%, and 115%, respectively. Tensile fracture surfaces of thermally stabilized PAN and the PAN/SWNT fibers exhibited brittle behavior and well distributed SWNT ropes covered with the oxidized matrix can be observed in the tensile fracture surfaces of the fibers. No de-bonding has been observed between unoxidized or the oxidized PAN matrix and the nanotube ropes. Higher strain to failure of the oxidized composite fiber as compared to that of the oxidized control PAN fiber also suggests good adhesion/interaction between SWNT and the oxidized matrix. Thermal stresses generated on the composite fiber during the oxidation process were lower than those for the control fiber. The potential of PAN/SWNT composite fiber as the precursor material for the carbon fiber has been discussed.     

Effect of processing parameters, applied pressure and temperature on the electrical resistivity of rubber-based conductive composites

Carbon black- and short carbon fibre (SCF)-filled conductive composites were prepared from ethylene vinyl acetate (EVA), ethylene propylene diene (EPDM) rubber and their 50:50 blend. The electrical resistivity of carbon black- and SCF-filled composites were measured under different conditions. The electrical conductivity of filled polymer composites is due to the formation of a continuous conductive network in the polymer matrix. These conductive networks involve specific arrangement of conductive elements so that the electrical paths are formed for free movement of electrons. It was found that electrical conductivity of filled conductive composites depends on different processing parameters like mixing time, rotor speed, mixing temperature, vulcanization time and pressure and service conditions like applied pressure and temperature. The results of different experiments have been discussed in light of break down and formation of the continuous conductive network.     

Single-source precursor route to carbon nanotubes at mild temperature

Carbon nanotubes were synthesized via a single-source precursor route at 500 °C, using iron carbonyl both as carbon source and catalyst. The X-ray power diffraction pattern indicates that the products are hexagonal graphite. Transmission electron microscope (TEM) images of the sample reveal carbon nanotubes with an average inner (outer) diameter of 30 nm (60 nm). High-resolution TEM indicates that fabrication of the carbon nanotube walls was composed of ca. 40 graphene layers. The Raman spectrum shows two strong peaks at 1587 and 1346 cm⁻¹, corresponding to the typical Raman peaks of graphitized carbon nanotubes. This method avoids the separation of raw material from solvent and simplifies the operation process. At the same time, the research provides a new route to large-scale synthesis of carbon nanotubes.     

Carbon nanotubes and onions from carbon monoxide using Ni(acac)2 and Cu(acac)2 as catalyst precursors

New catalyst precursors (copper and nickel acetylacetonates) have been used successfully for the synthesis of carbon nanotubes and onion particles from carbon monoxide. Catalyst nanoparticles and carbon products were produced by metal-organic precursor vapour decomposition and catalytic disproportionation of carbon monoxide in a laminar flow reactor at temperatures between 705 and 1216 °C. Carbon nanotubes (CNTs) were formed in the presence of nickel particles at 923-1216 °C. The CNTs were single-walled, 1-3 nm in diameter and up to 90 nm long. Hollow carbon onion particles (COPs) were produced in the presence of copper particles at 1216 °C. The COPs were from 5 to 30 nm in diameter and consisted of several concentric carbon layers surrounding a hollow core. The results of computational fluid dynamics calculations to determine the temperature and velocity profiles and mixing conditions of the species in the reactor are presented. The mechanisms for the formation of both CNTs and COPs are discussed on the basis of the experimental and computational results.     

Production and characterisation of vapour grown carbon fiber/polypropylene composites

Polymeric composites are widely used in the aircraft and automotive industries. Their high strength-to-weight ratio makes significant weight reduction possible. Beside this advantage, the polymer materials also offer a good corrosion resistance but the mechanical and electrical properties are not satisfactory. In order to increase these properties, vapour grown carbon fibers (VGCF) with high strength and metal-like electrical conductivity can be embedded in the polymeric matrix. To ensure a good adhesion between the fibers and the polymer matrix a functionalization of the chemically inert surface of the fibers is necessary.In the present research work oxygen-containing functional groups were introduced on the fiber surface through cold plasma treatment. Measurements of the fiber surface energy after plasma functionalization showed an enhancement of at least 50% of the initial value. The VGCF/PP composites with different amounts of VGCF were made through extrusion and injection molding. The results show that the degree of fiber surface functionalization and the fiber distribution and orientation in the polypropylene (PP) matrix may strongly influence the mechanical properties of the composite.     

Melt processing and mechanical property characterization of multi-walled carbon nanotube/high density polyethylene (MWNT/HDPE) composite films

High density polyethylene (HDPE) was used as the matrix material for a carbon nanotube (CNT) polymer composites. This combination of composite constituents has not been previously reported in the literature. Multi-wall carbon nanotube (MWNT)/HDPE composite films were fabricated using the melt processing method. The composite films with 0, 1, 3 and 5% nanotube content by weight were analyzed under SEM and TEM to observe nanotube dispersion. The mechanical properties of the films were measured by small punch test. Results show increases in the stiffness, peak load and work to failure for the composite films with increasing MWNT content.     

Mechanism for the growth of multiwalled carbon-nanotubes from carbon black

Multiwalled carbon-nanotubes have been grown from carbon black by solid-state transformation at the anode of a modified high-temperature arc-furnace without a catalyst. A mechanism for the solid-state transformation of carbon black into nanotubes is proposed. The migration of pentagon and heptagon defects present in carbon black to regions of high tensile-stress is key to the growth mechanism. The growth process can be broken into two stages. The basic mechanism for both stages is the same; only the source of the tensile stress that drives the nanotube growth differs. In the initial stage of growth the necks between carbon-black particles are lengthened into short nanotubes by thermal forces. Electrostatic forces present in the plasma of the high-temperature arc-furnace drive the subsequent extension of the short nanotubes to multiple-micron lengths.     

A dynamic continuum of nanostructured carbons in the combustion furnace

The presence of C60, carbon onions and higher fullerenes are confirmed in products from a carbon black combustion furnace. Acetylenic, C2, units participate in the formation and growth phases in the combustion flame. High-resolution TEM data clearly show fullerenes incorporated into the matrix of the carbon; MALDI mass spectral data show C60 and higher fullerenes in the soot itself. The toluene extracts of experimental carbons also contain C60.     

Compressive strength of three-dimensionally reinforced carbon/carbon composite

Compressive behavior of three-dimensionally reinforced carbon/carbon composite (3D-C/C) was examined from room temperature to elevated temperatures up to about 3000 K. Three-dimensionally reinforced C/C was found to have an inclination to induce kinks at the ends of specimens due to extremely low shear strength. In order to avoid this type of premature fracture and to conduct high-temperature tests, discussion was made on specimen geometry and testing procedure, and the combination of a dumbbell-shape specimen and test configuration without a supporting jig were found to be suitable for the present study. Using this set-up, the compressive strength of a 3D-C/C was evaluated as a function of temperature up to about 3000 K. The compressive strength of the 3D-C/C monotonically increased with the increase in temperature up to 2300 K, but decreased above this temperature. The strength enhancement was suggested to be caused by improvement in the fiber/matrix interfacial bonding, and the degradation over 2300 K was by softening of the matrix at high temperatures.     

The role of phosphorus in the growth of vapour-grown carbon fibres obtained by catalytic decomposition of hydrocarbons

Production of VGCF fibres from the decomposition of a methane-hydrogen mixture over metal particles is influenced by the support on which the particles have been laid. It was found that different as-received commercial graphite supports, according to their impurity content, could promote or inhibit the VGCF growth.Good yields of vapour-grown carbon fibres with a length up to 6 cm have been fabricated by catalytic decomposition of methane over particles obtained from Fe₃(CO)₁₂. Addition to the substrate of small amounts of phosphorus from a solution of H₃PO₄ in ethanol, followed by impregnation with Fe₃(CO)₁₂, was found to be effective in promoting the growth of VGCFs and increasing the yield. But increasing the amount of phosphorus over P/Fe ∼0.25 had an inhibiting effect on the growth of VGCFs. So the yield of VGCFs was optimized for a given phosphorus concentration.These phenomena are interpreted by the formation of Fe-P compounds which, depending on their formulae, lower or increase the melting point of the catalyst particles. According to the VLS theory, catalytic growth up to a macroscopic scale results from the liquid state of the catalyst.     

Multi-step purification of carbon nanotubes

An efficient purification procedure for multi-walled carbon nanotubes (MWNTs) synthesized by the floating catalyst method was discussed. The process involves ultra-sonication, heat treatment in hot water, bromination, oxidation and acid treatment. Most of amorphous carbon, multishell carbon nanocapsules as well as metal particles were successfully removed from the MWNT product. With this procedure, MWNTs with purity of more than 94% were obtained and the yield could approach 50%. It was found that bromination took an important role in the purification of MWNTs. Transmission electron microscopy, XPS and thermo-gravimetric analysis were used to evaluate the purification process of MWNTs. The mechanism of bromination on purification of the MWNTs was also discussed.