Strategies for control of ceramic powder synthesis by gas-to-particle conversion

Powder production by gas-to-particle conversion is a commonly used but poorly understood industrial process. The effects of process conditions, mixing, turbulence and chemical additives on coagulation- and condensation-controlled gas-to-powder conversion processes are elucidated. Strategies for control of average particle size, polydispersity and degree of agglomeration are outlined. Coagulation-controlled processes usually result in agglomerate powders of solid primary particles, while condensation-controlled processes result in nonagglomerate powders. In coagulation-controlled processes, the narrowest possible size distribution is set by the self-preserving limit for complete coalescence, σ g≈1.45. A possible way of further narrowing this distribution is by electrical charging of the aerosol. The size distribution broadens by shear-induced (turbulent) coagulation and by broad residence time distributions caused by mixing or gas recirculation. Sintering makes the primary particle size distribution narrower than that of the parent agglomerates. In principle, condensation-controlled processes may result in narrower size distributions than coagulation-controlled ones. Here, high reaction rates and low particle surface energies favor narrow size distributions.