SINTERING KINETICS AND TRANSPORT PROPERTY EVOLUTION OF LARGE MULTIPARTICLE AGGREGATES

Ultrafine (''nano''-) particles produced from highly supersaturated va pors or liquids are usually aggregated, often containing thousands of small 'primary' particles bound together in tenuous structures charact erized by mass fractal dimensions less than 3. Such aggregates have la rge initial surface area but are metastable with respect to more compa ct configurations. Available restructuring mechanisms include surface energy driven coalescence, which, in the case of viscous flow at high gas temperatures, is ultimately able to obliterate all evidence of the original (''primary'') particles. We here exploit the notion that, pr ovided an aggregate is sufficiently large, it can be treated like a sp atially non-uniform porous medium, undergoing finite-rate surface ener gy driven viscous flow sintering leading to final collapse to a single dense sphere. For this purpose, after a D-f congruent to const stage of sintering [associated with a corresponding increase in mean apparen t primary particle ('grain') size], we use an extension of the sinteri ng rate models of Mackenzie and Shuttleworth (1949) and Scherer (1977) , treating the material of the restructuring aggregate to be a Newtoni an viscous fluid. We predict and report here the time-dependent increa se in fractal dimension, D-f, and associated decreases in: aggregate o uter (maximum) radius, mobility radius, and changes in accessible surf ace area with dimensionless time [real time in multiples of the charac teristic sintering time, mu (R-1)(t=0)/sigma, where mu is the material 's viscosity (R-1)(t=0)is the effective initial grain radius and sigma the material surface tension]. In these units, we find that the total required coalescence time does not increase with N as sensitively as N-1/3... an important observation for processes involving very large a ggregates. With validation and the indicated extensions, our pseudo-co ntinuum methods are efficient enough to be used for estimating the mor phological-and transport property-evolution of entire populations of r estructuring aggregates. perhaps characterized by some nonseparable pr obability density function pdf(N,D-f,R-1), locally, in non-isothermal combustion-synthesis reactors.