MODELING THE FORMATION OF BORON-CARBIDE PARTICLES IN AN AEROSOL FLOW REACTOR

The formation of submicron crystals of boron carbide (B4C) by coagulation and sintering by the rapid carbothermal reduction of intimately mixed carbon-boron oxide powders in an aerosol flow reactor at temperatures above the boiling point of boron oxide is investigated. High heating rates (10(5) K/s) force rapid evaporation of boron oxide and suboxides from the precursor powder, resulting in its rupture and formation of boron carbide molecular clusters that grow to macroscopic particles by coagulation. Consequently, the formation and growth of B4C particles are described by simultaneous interparticle collision and coalescence using a two-dimensional particle-size distribution model that traces the evolution of both size and shape characteristics of the particles through their volume and surface area. In addition to the coagulation term, the governing population balance equation includes a coalescence contribution based on B4C sintering law. The predicted evolution of the two-dimensional particle-size distribution leads to a direct characterization of morphology as well as the average size and polydispersity of the powders. Furthermore, model predictions of the volume and surface area of boron carbide particles can be directly compared with experimental data of B4C specific surface area and an effective sintering rate of BC is deduced.