PARTICLE FORMATION AND GROWTH IN SINGLE-STAGE AEROSOL REACTORS

The feasibility of producing dense particles of a refractory material controlled by reaction of a vapor precursor has been examined experimentally and theoretically. Previous experiments that generated dense, spherical particles of silicon have been repeated with one modification. The peak temperature in the reactor was lowered to minimize sintering of any agglomerates formed early in the reactor. Agglomerate particles were generated with a peak reactor temperature of 1070 K. Raising the peak temperature to 1473 K resulted in nearly complete densification within a hot zone residence time of approximately 1 s. These results suggest that the dense particles previously reported resulted from sintering rather than vapor deposition. A simple model was used to probe the role of coagulation theoretically. Even with severely constrained reaction rates, a high concentration of fine particles was produced in the initial burst of nucleation. Coagulation plays a major role in the subsequent particle growth. The particles formed early in the reactor are efficient scavengers for clusters produced later, so only small clusters are formed after the initial nucleation burst for the conditions modeled. To suppress agglomeration, the number concentration produced by homogeneous nucleation must be reduced before entering the primary growth stage of the reactor. The use of a separate seed generation stage would be preferable to the single-stage reactor for particle growth by vapor deposition.