A multicomponent sectional model applied to flame synthesis of nanoparticles

Sodium/halide flame synthesis and encapsulation (SFE) is a promising technique for producing nonoxide materials. The reaction produces nanoparticles of the core product and a condensable by-product (e.g., NaF). In this research, the SFE process was studied numerically for a spherically symmetric non-premixed flame in a low-pressure atmosphere of sodium vapor. Experimentally, this flame can be established in microgravity. A transient flame code incorporating detailed chemistry and transport processes was developed to perform the investigation. In addition, a two-component sectional method was developed and integrated into the flame code to simulate aerosol dynamics, including coagulation and condensation. For the simulation, the reaction of CF₄ with Na was chosen such that solid carbon was the core product and NaF was the condensable by-product. The results show that near the reaction region, a large number of core particles form, yet condensation of the condensable species does not occur because of the high temperature. Slightly away from the reaction region toward the ambient, there is a narrow region within which rapid condensation of NaF occurs and a few large, heavily coated particles are formed. Most of the small core particles are not coated in this region, but instead are being scavenged by the large, heavily coated particles. Condensation is negligible further downstream because the concentration of the condensable species becomes too low. The simulation also shows that thermophoresis is important, especially at low pressure, to the distribution of particles and that the effect of sticking coefficient on NaF(1) is minimal. The effect of pressure also has been studied, and the results show that the mass fractions of carbon and NaF(1) are insensitive to pressure.