On coagulation mechanisms of charged nanoparticles produced by combustion of hydrocarbon and metallized fuels
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PROPERTIES OF SOLIDS
On Coagulation Mechanisms of Charged Nanoparticles Produced by Combustion of Hydrocarbon and Metallized Fuels A. M. Savel’ev and A. M. Starik* Baranov Central Institute of Aviation Motors, Moscow, 111116 Russia *e-mail: [email protected] Received April 11, 2008
Abstract—The contributions of van der Waals, Coulomb, and polarization interactions between nanometersized particles to the particle coagulation rate in both free-molecular and continuum regimes are analyzed for particle charges of various magnitudes and signs. Analytical expressions are obtained for the coagulation rate constant between particles whose interaction in the free-molecular regime is described by a singular potential. It is shown that van der Waals and polarization forces significantly increase the coagulation rate between a neutral and a charged particle (by a factor of up to 10) and can even suppress the Coulomb repulsion between likecharged particles of widely different sizes. PACS numbers: 36.40.Wa, 52.20.Hv, 79.40.+z DOI: 10.1134/S1063776109020150
1. INTRODUCTION Coagulation is the dominant process contributing to nanoparticle growth starting from primary clusters in various systems. The study of this process, initiated in the early 20th century [1], continues to this day [2–6]. There has been particular interest recently in mechanisms of cluster formation in aerodisperse systems containing ions and electrons, such as those created when nanoparticles are synthesized in gas discharges [6, 7], laser ablation [8, 9], or combustion of hydrocarbon and metallized fuels [10–12]. Such systems include the ionospheric plasma and upper atmospheres of planets [13, 14]. Systems of this kind also form behind leading shock waves in front of bodies moving at supersonic speeds in the atmosphere [15], in jet engine exhaust plumes [16, 17], and as a result of wall ablation in fusion reactors [18]. Even though these systems may have different parameters (temperature, ion and electron concentrations, residence time, etc.), the processes that determine their evolution have much in common. These processes include production of ions and electrons by an external ionizing agent (radiation, electric field) or chemiionization reactions, nucleation and surface growth of the dispersed phase by vapor condensation or chemical vapor deposition, cluster and particle charging by ion and electron attachment and thermoelectron emission (at high temperatures), and formation of a polydisperse system of neutral and charged particles via coagulation of clusters and particles. These systems contain not only neutral particles, but also positively and negatively charged ones whose charges are much larger than the electron charge.
Various mechanisms have been invoked to explain the growth of particles in dusty plasmas. First, it was noted that Coulomb interaction significantly increases the coagulation rate between oppositely charged particles and decreases the coagulation rate between likecharged particles [19]. In [20], an image potential was introduced to describe
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