Aerosol synthesis of ceramic particles by seed growth : analysis of process constraints

[摘要] ENGLISH ABSTRACT:Aerosol synthesis involves the formation of condensable product species by gas-phase reaction,and the simultaneous growth of particles by coagulation. For the production of ceramicparticles, reaction temperatures higher than 700 K are commonly used, and a maximumfusible particle size is observed.Coagulation-controlled growth yields spherical particles up to the maximum fusible size(approximately < 50 nm). Such particles coalesce rapidly and completely upon collisionwith other particles, whereas larger particles reach a meta-stable equilibrium for solid-statecoalescence. Agglomerates with weak Van der Waal's bonds between particles inevitablyform in the cooling/collection process.Coagulation of particles larger than the maximum fusible particle size yields agglomerateswith significant neck growth between the primary particles.Spherical ceramic particles in the order of 1 J-Lm are favourable precursors for bulk electronicapplications that require high purity. Such large spherical particles may possibly beproduced in conditions of seed growth, which involves the deposition of small newly formedclusters onto larger existing particles.The central focus of the present work is to evaluate whether spherical ceramic particlessignificantly larger than the maximum fusible size may be produced by seed growth. Theevaluation is done by modelling of process constraints and interpretation of published results.The modelling of constraints is based on a mathematical framework for comparison ofdifferent values of reactor design parameters. This framework comprises a simplified modelsystem, a typology of quantities, and isolation of a set of independent design parameters.Comparison is done on the basis of fixed initial (seed) and final (product) particle sizes.The reactor design framework is used to evaluate the hypothesis on spherical seed growth, byassessing whether a reactor can be designed that satisfies all the process constraints. Futureextension of the framework may allow optimisation for seed growth in general.The model system assumes laminar flow and isothermal conditions, and neglects the effectof reactor diameter on wall-deposition.The constraints are graphically represented in terms of the design parameters of initialreactant concentration and seed concentration. The effects of different temperatures andpressures on the constraints are also investigated.In a separate analysis, the suitability of turbulent flow for seed growth is assessed bycalculating Brownian and turbulent collision coefficients for different colliding species. Asturbulent intensity is increased, the seed coagulation rate is the first coagulation rate tobe significantly enhanced by turbulence, resulting in a lowering of the maximum seed concentrationallowed by the constraint for negligible seed coagulation. This tightening of aconstraint by turbulence is the justification for considering only laminar flow for evaluatingthe hypothesis on spherical seed growth.Quantitative application of the model of constraints, as well as experimental and modellingresults from the literature, did not demonstrate that significant spherical seed growthis possible without seed coagulation (agglomeration).As part of the conceptual effort in becoming familiar with aerosol reactor engineering, a simpletwo-mode plug-flow aerosol reactor model was developed, and verified with published results.This model has some novel value in that it translates the equations for aerosol dynamics intothe terminology of reactor engineering.

[发布机构] Stellenbosch University