THERMOKINETIC MODEL FOR THE FORMATION AND OXIDATION OF CARBON NANOFORMS

Abstract

A thermokinetic model for the formation and decomposition of layered carbon nanoforms (LCNFs) is proposed. The model is based on the fundamental dependence of the discrete temperature component in the CO (Boudoir–Mayer) disproportion reaction. It represents a superposition of reversible redox reactions of individual LCNFs in the range 820–1070 K. The superposition of the reactions has two thermokinetic components: fast (discrete) and slow (continuous). The velocity v_{red/ox C} of the continuous process is 10^–8–10^–7 mole/sec. The activation parameter Е_{а ох C} of the first-order reaction (smoldering) is defined as 35 ± 5 kJ/mole. The rate of rapid oxidation processes has an exponential temperature dependence and v_{red/ox C} ≈ 10^–6–10^–5 mole/sec in the range 1073–1473 K. The activation energies of oxidation (combustion) of most nanoforms are in the range ~173 ± 5 kJ/mole. The oxidation temperatures of the carbon nanoforms were determined. The following carbon nanoforms are mainly formed 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, tubular nanofibers at 1013 K, multi-walled nanotubes at 1033 K, and single-walled nanotubes at 1173 K. The carbon polymerization mechanism is free-radical. The catalytic basis of polymerization includes three types of primary paramagnetic carbon radicals with one to three free electrons. The radical concentrations look like a superposition of the Gaussian curves on the acceptor surface of the promoter in the range 820–1070 K. Primary radicals give rise to three types of nanoforms: spherical, lamellar, and tubular. The reactions are re-versible depending on the environment and temperature. When equilibrium precipitation of the LCNFs and “carbon sludge” is achieved, the reaction package is eventually converted to a Ƨ-curve similar to the temperature dependence of the partial pressure of CO. The chemical properties of nanoforms combine two types of topological interaction: peripheral hydrophilic and surface hydrophobic. Peculiarities of synthesis and up-stop-up oxidation procedures in the qualification and selective determination of carbon nanoforms in mixtures are discussed.