Gary G. Tibbetts

Why are carbon filaments tubular

Abstract Carbon whiskers (more commonly called filaments) grown above 900°C by decomposition of gaseous hydrocarbons on submicron catalytic particles are invariably tubular. Because the surface free energy of the (0001) basal plane of graphite is exceptionally low, the free energy required for filament growth is minimized when the outer surface is a curved basal plane. We consider the chemical potential of such an arrangement, and show that it is energetically favorable for the filament to create a hollow core rather than precipitate highly strained cylindrical planes of small diameter. Calculations based on this model give encouraging agreement with measured whisker inner diameters. Furthermore, the observed variation of the hollow core diameter along the whisker length may be a consequence of a weak maximum in the chemical potential.

Hydrogen storage capacity of carbon nanotubes, filaments, and vapor-grown fibers

Abstract We have studied the sorption of hydrogen by nine different carbon materials at pressures up to 11 MPa (1600 psi) and temperatures from −80 to +500°C. Our samples include graphite particles, activated carbon, graphitized PYROGRAF vapor-grown carbon fibers (VGCF), CO2 and air-etched PYROGRAF fibers, Showa-Denko VGCF, carbon filaments grown from a FeNiCu alloy, and nanotubes from MER Corp. and Rice University. We have measured hydrogen sorption in two pieces of equipment, one up to 3.5 MPa, and one to 11 MPa. The results so far have been remarkably similar: very little hydrogen sorption. In fact, the sorption is so small that we must pay careful attention to calibration to get reliable answers. The largest sorption observed is less than 0.1 wt.% hydrogen at room temperature and 3.5 MPa. Furthermore, our efforts to activate these materials by reduction at high temperatures and pressures were also futile. These results cast serious doubts on any claims so far for room temperature hydrogen sorption in carbon materials larger than a 1 wt.%.

Mechanical properties of vapor-grown carbon fiber composites with thermoplastic matrices

This article discusses the mechanical properties of vapor-grown carbon fiber (VGCF)/nylon and VGCF/polypropylene composites. Fibers in the as-produced condition yielded composites with marginally improved mechanical properties. Microscopic examination of these composites clearly showed regions of uninfiltrated fibers, which could account for the unsatisfactory mechanical properties. The infiltration of the fibers by both polymers was improved by carefully ball milling the raw fiber so as to reduce the diameter of the fiber clumps to less than 300 μm. Properties of composites made with ball-milled material were improved in every respect. VGCF reinforcement in nylon slightly improved the tensile strength and doubled the modulus, while VGCF in polypropylene doubled the tensile strength and quadrupled the modulus compared to unreinforced material. Moreover, the composites were sufficiently improved that differences in fiber surface preparation became important. For example, air-etched fibers and fibers covered with low concentrations of aromatics produced polypropylene composites with significantly better mechanical properties than did fibers whose surfaces were heavily coated with aromatics. Both the tensile strength and the modulus of the composites fabricated with clean fibers exceeded theoretical values for composites made with fibers randomly oriented in three dimensions, indicating that the injection-molding process oriented the fibers to some extent.

Role of sulfur in the production of carbon fibers in the vapor phase

Abstract Iron particles do not grow filaments in a methane atmosphere profusely enough to make a continuous reactor practical. Adding small quantities of sulfur to the iron vastly increases filament formation. We show that this is because the sulfur liquefies the iron particle, enhancing filament nucleation. With continued increases in sulfur, the number of filaments produced continues to increase, but quality, measured by length and straightness, decreases. We attribute this to higher sulfur concentration in the catalyst particle moving the melting point above the eutectic, thus decreasing the efficiency of filament lengthening.

Carbon fibers produced by pyrolysis of natural gas in stainless steel tubes

Carbon fibers of uniform diameter have been grown by pyrolysis of natural gas in type 304 stainless steel (18% Cr, 8% Ni) tubes at temperatures between 950 and 1075 °C. The method utilizes the circulation of wet hydrogen outside the growth tube in order to promote effective nucleation. Fibers 12 cm long having average moduli of 1.8×1011 Pa have been grown.

Vapor-grown carbon fibers: Status and prospects

Abstract Vapor-grown carbon fibers are produced by exposing a metal catalyst particle (usually Fe) of a few nanometers in diameter to a gas supersaturated in carbon. Under these conditions, the catalyst particles can produce rapidly lengthening carbon filaments of nanometer diameter. Macroscopic fibers may be obtained by vapor deposition of carbon on these filaments. Several different approaches are now being used to exploit this concept. In the first, Fe particles are deposited on a substrate on which the fibers are grown. This method can produce fibers of good uniformity, stiffness of 240 GPa (34 Msi), and tensile strength of 2.9 GPa (420 ksi). It has been researched in Japan, France, and the U.S.A. without the fibers being made available in commercial quantities. In a more recent conception, an organometallic containing iron dissolved in a liquid hydrocarbon is sprayed into a reactor under conditions where filaments can form. The filaments are of smaller diameter and length than the fibers grown on substrates, and their mechanical properties are inferior. However, because this process has a high reactor productivity and can be made continuous, several groups are now attempting to make these fibers commercially available. These fibers could find many applications in composite materials.

A new reactor for growing carbon fibers from liquid- and vapor-phase hydrocarbons

Abstract We describe an apparatus for continuous growth of carbon fibers designed to minimize thermophoretic and convective losses while maximizing nucleation of filamentous carbon. A high vapor pressure liquid hydrocarbon (hexane) dissolves a suitable organometallic source of iron catalyst particles, ferrocene. This solution is efficiently vaporized by incorporating it in a flowing gas stream containing some hydrocarbons and air. This feedstock flows through a tube of relatively low diameter and debouches into a larger tube maintained at 1100°C. A quantity of sulfur approximately equal to the quantity of iron catalyst material is vital to rich filament nucleation and is flowed in with the other reactants. The liquid feedstock flow rate should be rapid enough that it exactly saturates the gas stream to which it is added. This condition limits the throughput of reactants, and hence, the production rate of the fibers.

Role of nitrogen atoms in ``ion‐nitriding''

It is shown that pure Fe and 1020 steel are nitrided in an N2–H2 plasma principally by neutral nitrogen atoms.

Surface treatments for improving the mechanical properties of carbon nanofiber/thermoplastic composites

Nanofiber-matrix adhesion was studied after surface treating carbon nanofibers using a variety of methods. Among as-grown fibers, those produced with longer gas phase feedstock residence times were less graphitic but adhered to the polypropylene matrix better, giving improved tensile strength and modulus. A modest degree of oxidation was also found to increase adhesion to the matrix and increase composite tensile strength, while extended oxidation attacked the fibers sufficiently to decrease composite properties. Two chemical treatments were found to be ineffective in increasing tensile strength or modulus.

Physical properties of vapor-grown carbon fibers

Abstract Vapor-grown carbon fibers (VGCF) are produced by depositing a layer of pyrocarbon from the vapor phase on a catalytically grown carbon filament. This morphology determines many properties of the fiber, since the filament is more graphitic than the pyrocarbon. In this paper we compare VGCF produced by a continuous process with those grown on a substrate. Fibers having thicker pyrocarbon layers are less graphitic as measured by X-ray diffractometry, electron diffraction and Raman spectroscopy. The bulk density of the fibers, near 2.03 g/cm3, is relatively high for carbon fibers; it decreases slightly as the pyrocarbon thickness increases. The surface area of the fibers determined by N2 adsorption is not larger than the calculated geometric area, indicating that the surface is relatively smooth and free of pores. Each of these measurements indicates that fibers produced by a continuous process are comparable to those grown on substrates, with respect to graphitization and surface properties.