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

Human, Chris (2002-04)

Thesis (MScEng)--University of Stellenbosch.

Thesis

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 ceramic particles, reaction temperatures higher than 700 K are commonly used, and a maximum fusible 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 collision with other particles, whereas larger particles reach a meta-stable equilibrium for solid-state coalescence. Agglomerates with weak Van der Waal's bonds between particles inevitably form in the cooling/collection process. Coagulation of particles larger than the maximum fusible particle size yields agglomerates with significant neck growth between the primary particles. Spherical ceramic particles in the order of 1 J-Lm are favourable precursors for bulk electronic applications that require high purity. Such large spherical particles may possibly be produced in conditions of seed growth, which involves the deposition of small newly formed clusters onto larger existing particles. The central focus of the present work is to evaluate whether spherical ceramic particles significantly larger than the maximum fusible size may be produced by seed growth. The evaluation is done by modelling of process constraints and interpretation of published results. The modelling of constraints is based on a mathematical framework for comparison of different values of reactor design parameters. This framework comprises a simplified model system, 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, by assessing whether a reactor can be designed that satisfies all the process constraints. Future extension of the framework may allow optimisation for seed growth in general. The model system assumes laminar flow and isothermal conditions, and neglects the effect of reactor diameter on wall-deposition. The constraints are graphically represented in terms of the design parameters of initial reactant concentration and seed concentration. The effects of different temperatures and pressures on the constraints are also investigated. In a separate analysis, the suitability of turbulent flow for seed growth is assessed by calculating Brownian and turbulent collision coefficients for different colliding species. As turbulent intensity is increased, the seed coagulation rate is the first coagulation rate to be significantly enhanced by turbulence, resulting in a lowering of the maximum seed concentration allowed by the constraint for negligible seed coagulation. This tightening of a constraint by turbulence is the justification for considering only laminar flow for evaluating the hypothesis on spherical seed growth. Quantitative application of the model of constraints, as well as experimental and modelling results from the literature, did not demonstrate that significant spherical seed growth is possible without seed coagulation (agglomeration). As part of the conceptual effort in becoming familiar with aerosol reactor engineering, a simple two-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 into the terminology of reactor engineering.

AFRIKAANSE OPSOMMING: Aerosol sintese behels die gelyktydige chemiese vorming van 'n kondenseerbare spesie en die groei van partikels deur koagulasie. Temperature hoër as 700K word gewoonlik vir die aerosol sintese van keramieke gebruik. Partikels bo 'n sekere grootte kan nie saamsmelt nie. Koagulasie- beheerde groei lewer sferiese partikels tot en met die maksimum saamsmeltbare partikelgrootte (ongeveer < 50 nm). Wanneer sulke partikels bots, smelt hulle vinnig en volledig saam. Groter partikels bereik egter 'n meta-stabiele ewewig vir vastestof samesmelting. Agglomerate met swak Van der Waal's bindinge tussen partikels vorm onvermydelik wanneer die aerosol afgekoel word en die produk versamel word. Koagulasie van partikels groter as die maksimum saamsmeltbare grootte, lewer agglomerate met noemenswaardige nekvorming tussen primêre partikels. Sferiese keramiekpartikels in die ordegrootte van 1Mmis geskikte intermediêre produkte vir die vervaardiging van soliede eletroniese komponente waarvoor hoë materiaalsuiwerheid vereis word. Sulke groot sferiese partikels kan moontlik geproduseer word in toestande waar klein nuutgevormde partikels deponeer op groter bestaande partikels (saadpartikels). Die hoof-fokus van die huidige werk is om vas te stelof sferiese keramiekpartikels wat noemenswaardig groter is as die maksimum saamsmeltbare grootte, geproduseer kan word met die metode van saadpartikel-groei. Die moontlikheid word ondersoek deur 'n model van prosesbeperkings te maak, en deur gepubliseerde resultate te vertolk. Die model van prosesbeperkings is gegrond op 'n wiskundige raamwerk vir die vergelyking van verskillende waardes van reaktor ontwerp-parameters. Hierdie raamwerk bestaan uit 'n vereenvoudigde model-sisteem, 'n tipologie van verskillende soorte groothede, en die identifikasie van 'n stelonafhanklike ontwerp-parameters. Verskillende parameter-waardes word vergelyk vir dieselfde aanvanklike (saad) en resulterende (produk) partikelgroottes. Die reaktorontwerp-raamwerk word gebruik om die hipotese van sferiese saadpartikel-groei te evalueer, deur vas te stelof 'n reaktor ontwerp kan word wat aan al die prosesbeperkings voldoen. Mettertydse verfyning van die raamwerk kan dit moontlik geskik maak vir die optimering van saadpartikel-groei in die algemeen. Die model-sisteem is gebaseer op die aannames dat vloei laminêr en temperatuur konstant is, en die effek van reaktor-diameter op deponering op die reaktorwand word verontagsaam. Die posesbeperkings word grafies voorgestel in terme van oorspronklike reaktant-konsentrasie en saadpartikel-konsentrasie. Die effek van verskillende temperature en drukke op die prosesbeperkings word ook ondersoek. 'n Systaande ondersoek word gedoen oor die toepaslikheid van turbulente vloei vir saadpartikel- groei, deur botsings-koeffisiënte vir Brown-beweging en turbulensie te vergelyk. Wanneer turbulensie verhoog word, styg die koaguleringstempo van saadpartikels beduidend voordat ander koaguleringstempo's beduidend toeneem. Dit noodsaak 'n verlaging in die maksimum toelaatbare saadpartikel-konsentrasie om saadpartikel-koagulasie te verhoed. Hierdie verskerping van 'n prosesbeperking deur turbulente vloei, is die rede hoekom slegs laminêre vloei beskou word in die evaluering van die hipotese van sferiese saadpartikel-groei. Kwantitatiewe toepassing van die model van beperkings, asook eksperimentele en modellerings- resultate vanuit die literatuur, het nie getoon dat noemenswaardige groei van sferiese keramiekpartikels verkry kan word sonder saadpartikel-koagulasie (agglomerasie) nie. As deel van die proses om aerosol reaktor-ingenieurswese konsepsueelonder beheer te kry, is 'n eenvoudige twee-modus propvloei aerosol reaktor modelontwikkel. Die resultate van die model is bevestig deur vergelyking met gepubliseerde resultate. Hierdie model het die nuwigheid dat dit die vergelykings vir aerosol dinamika uitdruk in die terminologie van reaktor-ingenieurswese.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/52635
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