This review was undertaken to evaluate existing information about the fundamentals and engineering aspects related to particle formation from the gas phase.
What is Known:
* Particle production mechanisms in the gas phase are fairly well understood for dilute systems, but not for concentrated systems.
* The three major mechanisms are nucleation, condensation and coagulation.
* Coagulation is the most detrimental mechanism as it can lead to increased polydis- persity and undesired aggregation. Control of coagulation is essential for powder manufacture.
* In dilute systems, the average primary particle size can be controlled by the correct choice of available vapor and the number of nuclei produced.
* The width of the particle size distribution is determined by the residence time and temperature distribution of both the reactants and the condensing vapor.
* Coagulation rate is a function of the particle concentration, relative particle motion and turbulence. Coagulation leads to broader size distributions.
* Particle shape depends on the temperature and the environment.
* Primary particle shape can be spherical or facetted depending on the precursors. Continued gas reactions can produce porous particles, the porosity being eliminated by high temperature.
* Aggregates and agglomerates have differing morphologies depending on such factors as field gradients and particle dielectric constants.
* Gas produced particles are very pure, and purity can be ensured by the use of inert gas sheaths to prevent wall contact.
* Powder yields can be high - depending on the reactor type - because particle produc- tion can be continuous.
* The product quality and economics are often a trade-OR between reactor throughput and the acceptable degree of agglomeration. What is Unknown
* The relative importance of reactor design, growth process competition and coagulation suppression are not well understood for concentrated systems.
* The importance of additives , applied fields and reactor design criteria have not been widely studied.
* Dilution systems are not widely applied because of the additional gas volume involved in the final gas solid separation process, but strategic dilution, i.e. dilution at key reactor locations or at key residence times has not been explored and might provide a solution with minimal extra gas volume.
* Sensors for monitoring real time size and concentration information are not available for high solids gas loadings and hostile environment.
* Mehtods of producing pure non-aggregated powders of size distribution greater than 0.5 micrometer on high concentration gas systems are not available .
What is Needed
A fundamental research program which will :
* study the relative importance and interplay of the three major mechanisms using a simple design of laboratory reactor.
* study the effect of reactor geometry and design on particle size distribution, aggregation, purity and yield.
* develop a suitable reactor to study the role of additives and strategic dilution. With the overall objective of extending the useful size range of gas/solid formation processed to coarser sixes while maintaining high yield, purity, morphology and no aggregation.