Thermokinetic Model for the Formation and Oxidation of Carbon Nanoforms
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THERMOKINETIC MODEL FOR THE FORMATION AND OXIDATION OF CARBON NANOFORMS V.V. Garbuz,1,2 L.M. Kuzmenko,1 V.A. Petrova,1 and T.A. Silinska1 UDC 661.66+541.1+546.26+543.7 A thermokinetic model for the formation and decomposition of layered carbon nanoforms (LCNFs) has been proposed. The model relies on the fundamental dependence of discrete temperature component in the CO (Boudouard–Meyer) disproportionation reaction. Reversible reduction– oxidation reactions for individual LCNFs superpose in the range 820–1070 K. The superposition of reaction package has two thermokinetic components: rapid (discrete) and slow (continuous). The rate vred/ox C of the continuous process is 10–8–10–7 mole/sec. The activation parameter Ea ox C of the first-order reaction (smoldering) has been determined as 35 ± 5 kJ/mole. The rate of rapid oxidation processes exponentially depends on temperature, and vred/ox C is 10–6–10–5 mole/sec in the range 1073–1473 K. The activation energies for oxidation (combustion) of most nanoforms are in the range ~173 ± 5 kJ/mole. The oxidation temperatures for carbon nanoforms have been found. The following carbon nanoforms are mainly synthesized or oxidized in a CO or oxygen atmosphere: nanoonions at 823 K, graphite nanoplatelets at 873 K, stacked nanofibers at 923 K, conical nanofibers at 973 K, coiled nanofibers at 1013 K, multiwall nanotubes at 1033 K, and single-wall nanotubes at 1173 K. Carbon polymerization is driven by free radicals. The polymerization is catalytically promoted by three types of primary paramagnetic carbon radicals with one to three free electrons. The concentrations of radicals represent superposed Gaussian curves on the promoter’s acceptor surface in the range 820–1070 K. Primary radicals give rise to three nanoforms: spherical, lamellar, and tubular. The reactions are reversible as a function of environment and temperature. When equilibrium precipitation of the LCNFs and ‘carbon sludge’ is achieved, the reaction package is eventually converted to a S-shaped curve similar to the temperature dependence of CO partial pressure. The chemical properties of nanoforms combine two topological interactions: peripheral hydrophilic and surface hydrophobic. The synthesis and upstop-up oxidation procedures in the qualification and selective determination of carbon nanoforms in mixtures are discussed. Keywords: thermokinetic model, reversible reactions, layered carbon nanoforms.
INTRODUCTION Layered carbon nanoforms (LCNFs) synthesize in nature as carbon black through CO disproportionation when carbon-containing fuel is burnt in conditions of local oxygen deficiency at 820–1070 K. The synthesis of
1Frantsevich
Institute for Problems of Materials Science, National Academy of Sciences of Ukraine, Kyiv,
Ukraine. 2To
whom correspondence should be addressed; e-mail: [email protected].
Translated from Poroshkova Metallurgiya, Vol. 59, Nos. 3–4 (532), pp. 30–41, 2020. Original article submitted September 10, 2019. 1068-1302/20/0304-0141 2020 Springer Science+Business Media, LLC
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