Effect of the size and structure of soot particles synthesized during pyrolysis and combustion of hydrocarbons on their optical properties

The effect of the size and structure of soot particles synthesized during combustion in a flat premixed flame of hydrocarbons and during pyrolysis behind shock waves on their optical properties is analyzed. Experiments are conducted to measure the optical properties and structure of soot particles in a propylen–air flame and during the pyrolysis of mixtures of $3\%$ of acetylene, $5\%$ of ethylene, and $2\%$ of propylene in argon behind shock waves in an shock tube. The function of the refractive index of soot particles and its change in the visible and near-IR spectral ranges are obtained using the laser-induced incandescence method. The mean soot particle sizes and parameters of the internal structure of soot particles are measured using transmission electron microscopy. Based on the analysis of the results obtained in this study and those available in the literature, it is found that, as the size of soot particles increases, the refractive index function for them at a wavelength of $1064$ nm increases by a factor of $2.5~($from $0.2$ to $0.5)$ and depends on the synthesis conditions and type of hydrocarbon. The ratio of the refractive index at a wavelength of $1064$ nm to its value at $532$ nm with an increase in the average particle size increases slightly or remains approximately constant $($in the range of $0.8$ to $1.15)$ for most soot particles or increases more significantly $($from $1.1$ to $1.5)$ in the case of acetylene soot. The observed significant changes in the optical properties are correlated to a decrease in the average distance between parallel graphene planes in the structure of growing soot particles and an increase in the number of adjacent parallel planes. These structural changes characterize the graphitization process of soot particles as they grow.