Fluidic and thermal modeling for the high production rate synthesis of high quality nanoparticles Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/np193d29b

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  • Advances of colloidal nanomaterials for societal benefit have been hampered by the high cost and low quality of nanoparticles (NPs). The production of high quality nanoparticles within colloidal suspension has two related concerns; (1) current high production rates methods of synthesizing nanoparticles result in a larger range of particle size, which requires expensive and time consuming separation steps resulting in high costs; and (2) the thermal and concentration gradients within batch processes used to scale-up colloidal NP synthesis results in products of varying sizes. Continuous flow microreactors provide a means to minimize these gradients during the synthesis of colloidal NPs thereby providing the potential to produce size-controlled suspensions at higher production rates compared to conventional batch reactors. In this dissertation, a number of microreactor mixing strategies are investigated. In addition, efforts are made to model the thermal profile caused by the conversion of microwave energy within a continuous flow scenario. Based on learnings, efforts are made to redesign flow applicators to maximize energy absorption with minimal thermal gradient. Within this dissertation, concentration gradients are controlled through the use of different mixing schemes within the various microreactor setups. It is demonstrated that the ability to control the mixing characteristics provides the ability to tune the NP size. T-mixing, interdigital mixing and reverse oscillatory flow mixing are all modeled and evaluated for mixing time. Mixing quality and mixing time metrics are defined and used for comparison of these methods. The scalability of these methods is explored in order to show methods which can maintain small particle size distributions at high production rates. In particular, a new reverse oscillatory flow (ROF) mixing system is developed for high rate NP synthesis. The relatively large size (460 µm by 152 mm channel) produces high production rates of nanoparticles while maintaining quality mixing through a novel mixing method. The ROF system is shown to produce CdS nanoparticles at a production rate of 115.7 g/hour with a coefficient of variation down to 19%. The size distributions of this method are comparable to other methods with production rates from 1% to 10% of the ROF method. In colloidal NP syntheses, thermal gradients are controlled by the time scale for heating. Here, a single mode microwave system is designed and developed for rapidly heating the reacting flux. Rapid heating minimizes the thermal gradients within the solution during synthesis, thereby shortening the nucleation time scale and providing opportunity for burst nucleation. A model is developed to simulate microwave heating which is verified over a range of operating parameters; flow rates (15 to 40 mL/min), microwave power (150 to 300 Watts) and salinity (1 to 5 g/L). Experimental results show model predictions of the temperature profile within 16.8% for all cases considered (averaged absolute error reported).
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