STRATEGIES FOR CONTROL OF CERAMIC POWDER SYNTHESIS BY GAS-TO-PARTICLECONVERSION

Powder production by gas-to-particle conversion is a commonly used but poorly understood industrial process. The effects of process conditio ns, mixing, turbulence and chemical additives on coagulation- and cond ensation-controlled gas-to-powder conversion processes are elucidated. Strategies for control of average particle size, polydispersity and d egree of agglomeration are outlined. Coagulation-controlled processes usually result in agglomerate powders of solid primary particles, whil e condensation-controlled processes result in nonagglomerate powders. In coagulation-controlled processes, the narrowest possible size distr ibution is set by the self-preserving limit for complete coalescence, sigma(g) approximate to 1.45. A possible way of further narrowing this distribution is by electrical charging of the aerosol. The size distr ibution broadens by shear-induced (turbulent) coagulation and by broad residence time distributions caused by mixing or gas recirculation. S intering makes the primary particle size distribution narrower than th at of the parent agglomerates. In principle, condensation-controlled p rocesses may result in narrower size distributions than coagulation-co ntrolled ones. Here, high reaction rates and low particle surface ener gies favor narrow size distributions.