Production of Nanoparticles in Thermal Plasmas: A Model Including Evaporation, Nucleation, Condensation, and Fractal Aggregation Norma Yadira Mendoza Gonzalez, Mbark El Morsli, and Pierre Proulx (Submitted March 20, 2008; in revised form June 20, 2008) In this work a coupled model for the production of nanoparticles in an inductively coupled plasma reactor is proposed. A Lagrangian approach is used to describe the evaporation of precursor particles and an Eulerian model accounting for particle nucleation, condensation, and fractal aggregation. The models of the precursor and nanoparticles are coupled with the magneto-hydrodynamic equations describing the plasma. The purpose of this study is to develop a model for the synthesis of particles in a thermal plasma reactor, which can be used to optimize industrial reactors. The growth of aggregates is considered by introducing a power law exponent D f . Results are compared qualitatively and quantita- tively with existing experimental data from plasma reactors at a relatively large laboratory scale. The results obtained from the model confirm the previously observed importance of the quench strategy in defining the morphology of the nanoparticles. Keywords CFD modeling, fractal particles, ICP plasmas, method of moments, nanoparticle synthesis 1. Introduction Nanoparticles are a very important building block of the new nanotechnologies because of their often unusual optical, mechanical, catalytic, and electrical properties, and remarkably high specific surface areas (Ref 1). The processes used for generating nanoparticles play an important role on the product purity, size distribution, particle size, and particle morphology. Among the existing processes, high-frequency inductively coupled plasma (ICP) is an attractive method for synthesizing (Ref 2, 3). Good quality of particles including narrow size distribu- tion, spherical shape, pure powder, and high production rates are the main features of the powders produced using this technology. Much improved control over temperature and gaseous mixtures over conventional flame technolo- gies are also a very important feature of ICPs. Detailed modeling of nanoparticle formation in plasma reactor has advanced in recent years to a degree that comparison with experimental results has become quantitative, as reported in the works of Ref 4-8. The experimental study of Goortani et al. (Ref 9) showed that the presence of agglomerates was largely controlled by the quench position and geometry of the reactor. In fact, in most cases of particle synthesis processes, there is the formation of agglomerates of individual spherical primary particles (Ref 10). The formation of particles consists of an initial phase of coalescent growth, where coagulation with small particles caused by high particle formation and rapid surface growth makes the particles to grow into near-spherical ‘‘primary’’ particles. The coales- cent regime is followed by particle aggregation, when the particles take the form of fractal aggregates (Ref 11). The volume of the agglomerates can be related to their radius of gyration (R g ) by a power law, which can be used to explain the relationship between the mass and size. The exponent of this power law is called the mass fractal dimension (MFD), D f (Ref 12). The MFD D f determines the collision diameter of the agglomerates and subse- quently their growth rate (Ref 13, 14). In addition, the dynamic behavior of these agglomerates is considerably different from their spherical counterparts. Several investigations of nonspherical particles in high-tempera- ture synthesis of materials where agglomerates of indi- vidual spherical (primary) particles are formed have been reported (Ref 15-18). Till date, different models have been used to explain the nature of the nanopowders produced in thermal plasma technology, where the nanoparticles are regarded as ideal spheres (Ref 6, 19, 20). In the present work, based on the ideas of Bilodeau (Ref 21), a Moment model is used to describe the initial evolution of the population of aerosol in the coalescent regime. To describe the aggregation regime, two other Moments equations are Norma Yadira Mendoza Gonzalez, Mbark El Morsli, and Pierre Proulx, Laboratoire de Mode ´ lisation Mathe ´ matique des Proce ´de ´s Chimiques OPPUS, Chemical Engineering Department, Univer- site ´ de Sherbrooke, Sherbrooke, QC, Canada J1K 2R1. Contact e-mail: norma.mendoza@usherbrooke.ca. JTTEE5 17:533–550 DOI: 10.1007/s11666-008-9209-x 1059-9630/$19.00 Ó ASM International Journal of Thermal Spray Technology Volume 17(4) December 2008—533 Peer Reviewed