David Jacques

Continuous production of aligned carbon nanotubes: a step closer to commercial realization

High-purity aligned multi-walled carbon nanotubes MWNTs were synthesized through the catalytic decomposition of a ferrocene-xylene mixture at ; 6758C in a quartz tube reactor and over quartz substrates, with a conversion of ; 25% of the total hydrocarbon feedstock. Under the experimental conditions used, scanning electron microscope images reveal that the MWNT array grows perpendicular to the quartz substrates at an average growth rate of ; 25 mmrh. A process of this nature which does not require preformed substrates, and which operates at atmospheric pressure and moderate temperatures, could be scaled up for continuous or semi-continuous production of MWNTs. q 1999 Elsevier Science B.V. All rights reserved.

Fabrication of Carbon Multiwall Nanotube/Polymer Composites by Shear Mixing

The dispersion of nanotubes in polymer matrices has been investigated as a means of deriving new and advanced engineering materials. These composite materials have been formed into fibers and thin films and their mechanical and electrical properties determined. The nanotube concentration at which conductivity was initiated (the percolation threshold) varied with host polymer. In poly(propylene), this was as low as 0.05 vol.-%, while higher concentrations were required for polystyrene and particularly for ABS. There was a small increase in elastic modulus and decrease in tensile strength at low nanotube loading, but as the concentration was increased there was a progressive increase in both strength and stiffness.

NANOTUBE COMPOSITE CARBON FIBERS

Single walled carbon nanotubes (SWNTs) were dispersed in isotropic petroleum pitch matrices to form nanotube composite carbon fibers with enhanced mechanical and electrical properties. We find that the tensile strength, modulus, and electrical conductivity of a pitch composite fiber with 5 wt % loading of purified SWNTs are enhanced by ∼90%, ∼150%, and 340% respectively, as compared to the corresponding values in unmodified isotropic pitch fibers. These results serve to highlight the potential that exits for developing a spectrum of material properties through the selection of the matrix, nanotube dispersion, alignment, and interfacial bonding.

Purification and structural annealing of multiwalled carbon nanotubes at graphitization temperatures

In this work, we present a systematic study of the effects of graphitization on the structural perfection of multiwalled carbon nanotubes. High purity nanotubes were produced by a low temperature CVD method and subsequently annealed at temperatures between 1600 and 3000°C. The nanotubes were characterized for chemical purity, interlayer spacing, and defect healing. The graphitization procedure was found to remove residual metal catalyst in the nanotubes and reduce the wall defects as reflected in a reduced interlayer spacing between the graphene shells. Graphitization presents a low-cost, commercially viable method of purifying and ordering multiwall carbon nanotubes.

In situ-grown carbon nanotube array with excellent field emission characteristics

In situ catalytic thermal decomposition method was used for producing aligned multiwalled carbon nanotubes (MWNTs) in bulk quantities on stable and electrically conducting substrates. Very low turn-on electric fields of 0.75 V/μm and low threshold fields of ∼1.6 V/μm (for current density of 10 mA/cm2) were obtained from the MWNT arrays grown on TiN substrate. Furthermore, large emission current densities of 1–3 A/cm2 were obtained at reasonably low fields of less than ∼8 V/μm. These enhanced emission properties are tentatively attributed to the oriented and high-density nature of the emitting carbon nanotube structure and the high-conductivity, stable nature of the TiN substrate onto which the nanotubes are attached.

Synthesis of isotropic carbon fibers and activated carbon fibers from pitch precursors

The influence of pitch precursor composition on the formation and properties of isotropic pitch fibers from non-conventional precursors has been examined. Changes in fiber weight that occur during the oxidative stabilization of green fibers are inversely related to the observed axial contraction. The weight gain upon stabilization also increases, and the contraction decreases with increasing pitch carbon content and aromaticity while the opposite occurs with increasing hydrogen and hetero-atom (H, N, O and S) content. Similar trends are found for fiber carbonization. The combined effects of stabilization and carbonization give a fiber yield of between 50 to 86% of the green fibers, with axial contractions of 12 to 28%, the highest yield corresponding to the smallest contraction. The net yield increases with pitch carbon content and aromaticity, and decreases with hetero-atom content, while the overall axial contraction decreases. The fiber tensile strength was found to increase with precursor carbon content, carbon yield and aromaticity. The activation rate of the derived fibers increased with increasing heteroatom content, especially oxygen content. While most fibers were microporous upon activation, fibers from shale oil and the sub-bituminous coal extract, developed a more mesoporous structure.

Observation and formation mechanism of stable face-centered-cubic Fe nanorods in carbon nanotubes

The crystallographic structure and orientation of iron nanoparticles present in carbon nanotubes (CNTs) was studied when iron was used as a catalyst. It was found that while most of the nanoparticles encapsulated inside the CNTs had the expected α-Fe (body-centered-cubic) phase, a significant number of them formed and retained the γ-Fe (face-centered-cubic) phase that is not the normal bulk phase at room temperature (nor even expected to form at the growth temperature used). It was also found iron particles at the tips of the nanotubes were either α-Fe or cementite (Fe 3 C). On the basis of these observations and thermodynamics, a mechanism for the formation of these particles and insights into CNT growth is proposed.

Visible photoluminescence from ruthenium-doped multiwall carbon nanotubes

Visible photoluminescence at 515 nm of ruthenium-doped multiwall carbon nanotubes, fabricated on quartz substrates using a chemical vapor deposition technique, is reported. The well-aligned nanotubes serve as templates for the luminescent, residual ruthenium–iron catalyst particles contained within the nanotubes, restricting the particle size to about 10 nm. The synthesis technique can be readily extended to other luminescent dopants; moreover, since nanotube arrays can be readily grown from patterned substrates, nanotube-based optoelectronic devices may be achieved.

Aspect Ratio and Loading Effects of Multiwall Carbon Nanotubes in Epoxy for Electrically Conductive Adhesives

Isotropic conductive adhesives (ICAs) filled with metal particles are commercially available as alternatives to solder joining in electronic packaging. Replacing metal fillers with multiwall carbon nanotubes (MWCNTs) offers the potential benefits of being corrosion resistant, high strength and lightweight. Traditional metal filled ICAs require high metal loading to ensure electrical conductivity, which may cause problems with respect to reliability and strength to weight ratio. The ultra-high aspect ratio and surface area of multiwall carbon nanotubes induce a low percolation threshold of less than 0.25 wt% in epoxy. MWCNTs dispersed in epoxy increase the thermal diffusivity of the polymer by a factor of 2 to 3 and decrease the volume resitivity to less than 10 Ω cm for loadings up to 12 wt%. Lap shear strength decreases with increased MWCNT loading. The dependences of electrical, thermal and mechanical properties on loading and aspect ratio of MWCNTs are reported, and the electrical conduction mechanism is discussed.

Tearing open nitrogen-doped multiwalled carbon nanotubes

Reductive alkylation of N-MWNTs with Li/NH3 results in fracturing of the nanotubes, ripping channels that breach the central core and generating significant new pore volume.