Thesis (MScEng)--University of Stellenbosch. / 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.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:sun/oai:scholar.sun.ac.za:10019.1/52635 |
| Date | 04 1900 |
| Creators | Human, Chris |
| Contributors | Bradshaw, S. M., Stellenbosch University. Faculty of Engineering. Dept. of Process Engineering. |
| Publisher | Stellenbosch : Stellenbosch University |
| Source Sets | South African National ETD Portal |
| Language | en_ZA |
| Detected Language | Unknown |
| Type | Thesis |
| Format | 188 p. : ill. |
| Rights | Stellenbosch University |
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