Electrical, magnetic and structural characterization of fullerene soots

Abstract

Although it is some five years since fullerenes were extracted in macroscopic quantities from the black, superficially amorphous sooty deposits produced by a carbon arc under helium, little is known in detail about the structure of the deposit or its electrical and magnetic properties. Here we provide evidence that this deposit, known as fullerene soot, is composed of defective networks of carbon atoms which do not have all valencies satisfied. We have studied these soots, before and after thermal annealing, using x-ray and electron diffraction, electron spin-resonance (ESR) spectroscopy, infra-red transmission and measurements of electrical conductivity. We find that localized states associated with such dangling bonds are removed from the soot on annealing and this process is accompanied by an ordering transition which modifies the electrical and magnetic properties. The fullerene soot particles appear to be encapsulated aggregates of highly defective carbon `onions'. Such metastable defective networks undergo a subtle ordering processes upon heat treatment which is accompanied by a rise in the electrical conductivity and a loss of paramagnetism due to the elimination of unsatisfied carbon atom valencies. Electrical conductivity and infra-red transmission measurements indicate that the centre of these `onions' is graphitic, with metallic properties. The temperature dependence of the electrical conductivity suggests that charge transport in both annealed and unannealed materials occurs by tunnelling between metallic islands in the sample. The ESR linewidth, arising from the spin centres in fullerene soots, is not significantly changed by exposure to oxygen. This suggests that the free radical centres in fullerene soots are extremely efficiently isolated from the atmosphere - presumably by encapsulation. This behaviour contrasts with that of amorphous carbons prepared by thermal decomposition of organic materials (chars). The ESR g-factors of the fullerene soots are lower than those of chars, which suggests that the radicals in fullerene soots have strong sigma character due to unsatisfied -type valencies. In this paper, a plausible structure and associated annealing mechanism for the fullerene soot is presented based on these experimental observations. .

Conductometric Soot Sensor for Automotive Exhausts: Initial Studies

“…This is due to the increased conductivity of the soot with increasing temperature, an effect that has already been observed in literature when the coke formation in fixed bed catalysts has been in-situ determined [ 21 ]. Investigations on fullerene soot also depict increasing conductivity at higher temperatures [ 22 ]. As soon as the soot ignition temperature is reached, the thin soot film is oxidized and the resistances increase rapidly.…”

Comparative Study of Different Methods for Soot Sensing and Filter Monitoring in Diesel Exhausts

“…Since soot is known to be electrically conductive [21,22], the field strength gets attenuated. This leads to a decrease of the averaged (in a certain frequency range) transmission value | S 21 |.…”

Fullerene black: Structure, properties and possible applications

“…The ESR signal of the fullerene black calcined in a vacuum at 1000°C, in contrast to the signal of amorphous carbon obtained by decomposition of sulfanilic acid, is not broadened under the influence of oxygen [28,29]. The thermal (500-2400°C) processing of fullerene black leads to the formation of multi-layer particles of "onion" type, but no graphitization is observed [30,31].…”

“…Graphite and graphitized particles are oxidized at higher temperatures [1,8,9]. The oxidation of fullerene black was studied recently [18][19][20][21][22][23][24][25][26][27][28][29], but these works were largely of phenomenological character. Nitric oxide oxidizes fullerene black at 1273.C with the formation of a mixture of N2O, NO2, N2, CO, and CO2 [25].…”