Formation of carbon nanoparticles from the gas phase in shock wave pyrolysis processes

This review deals with the most recent achievements in experimental investigations of the process of carbon nanoparticle formation, at pyrolysis of various carbon bearing species behind shock waves. The diverse diagnostic methods of these processes are described; special attention is given to new methods for measuring the current sizes and optical properties of particles and the temperature of the reacting mixture using time resolved laser-induced incandescence (LII), and IR emission-absorption spectroscopy. The main part of the review provides critical analysis of the numerous results of the kinetics of particle formation at various temperatures, pressures and concentrations of carbon. Particular emphasis is placed on the results obtained by pyrolysis of hydrogen free precursors. It is shown that recent measurements of size dependence of the optical properties of particles, actual temperature of the mixture during pyrolysis of initial substances, and the subsequent growth of nanoparticles require a serious revision of current conceptions regarding the temperature dependence of particle yield and growth rate. Based on this analysis, unified regularities in these processes, with various temperatures and types of initial substances, are suggested. The last section of the paper contains a short review of the methods for modeling the processes of carbon nanoparticle formation in shock waves. Emphasis is placed on the necessity for the elaboration of more general models describing the detailed changes in particle properties during the growth process and the unified regularities of particle growth from hydrocarbons and hydrogen-free precursors as determined in experiments.