Graphitic Structures by Design

Abstract

It is shown that self-supporting graphitic structures of specific shape can be grown in a variety of forms, from nanoscale to macroscale, on metal templates, in a fuel-rich mixture of ethylene and oxygen at temperatures between 750 and 900 K. The evidence presented suggests graphite can be grown in any shape created from catalytic metals (e.g., Ni) under the proper conditions of temperature and gas composition. Structures produced include macroscale bodies, centimeters in dimension, composed of micrometer-scale graphite elements such as graphite "foam" and regular graphite "lattices". Nanoscale hollow graphite spheres were also produced. The production rate in the apparatus employed was roughly shown to be 1 layer/s and was steady with time over several hours. The process of producing self-supporting bodies generally produces hollow graphite structures, as the underlying metal template must be removed by acid following the completion of graphite growth. The process is believed to be possible only in an environment, such as combustion, in which a high concentration of particular radical species is present in the vicinity of the template surface. The following process is postulated: (i) a single layer of graphite is formed from gas-phase radicals by the catalytic action of the metal template, (ii) additional graphite growth is "autocatalytic" and occurs via the decomposition of radicals on the surface and the incorporation of "free" carbon atoms, or other radical fragments, into "edge sites" on the graphite surface.

Scaling up the Fabrication of Mechanically-Robust Carbon Nanofiber Foams

“…That is, true precursors to carbon growth are homogenously-formed radicals that are generated by combustion processes [26,29,45]. As demonstrated in earlier publications [35,46], the rate of carbon growth is a strong function of the placement of the catalysts relative to the residence time of the reactant mixture.…”

“…This design was based on the assumption that the mechanism of carbon growth is a type of chemical vapor deposition (CVD), a Fibers 2016, 4, 9 6 of 14 mechanism associated with many forms of carbon growth from vapor phase hydrocarbons at elevated temperature [26,[30][31][32][33]41,42]. The CVD model assumes that carbon structures, including filaments, will grow on proper metal catalysts, particles, foils, etc., above the experimentally-determined breakdown temperature, generally between 500 and 1200˝C, of the organic precursor molecule, and that the process will continue until the hydrocarbon is fully depleted.…”

“…As demonstrated in the many studies of carbon growth from fuel-rich hydrocarbon mixtures, the so-called Graphitic Structures by Design (GSD) technology, carbon grows catalytically from radical species formed by homogeneous combustion [26][27][28][29]. To understand this mechanism it is useful to contrast GSD with fiber growth in oxygen-free environments.…”

Physicochemical Properties of Carbon Materials: A Brief Overview

“…When transition-metal catalyst particles are involved in carbon growth (see Figure 1.6), the mechanistic aspects are much more complex and also more controversial. In a particularly thought-provoking recent contribution, Phillips and co-workers [64] argue that the mechanism according to which "the metal acts as a template for carbon atoms such that as they diffuse to the metal surface they can organize into a low-energy form" is "at best a vague model" because, for example, it does not explain why graphite is "formed several atomic layers from the metal surface." Indeed, the absence of a detailed discussion on the origin of, or evidence for, curvature-even in presumably mechanistic papers published in reputable journals-is too common and thus puzzling.…”

Novel Growth of Multiscale Carbon Nanofilaments on Carbon and Glass Fibers

“…22 By using the unique characteristics of the GSD approach (relatively low temperature, standard atmospheric pressure), a theoretical assumption of prior work is challenged which is that synthesis of carbon nanofibers (nanotubes or otherwise) occurs due to thermal decomposition of molecules. Furthermore, a new theory is formulated that the actual driving force of nanofiber synthesis is the creation of radical species created by the combustion process.…”