Date of Award
Doctor of Philosophy (PhD)
Aerosol science and technology has enabled the material synthesis of ‘good’ nanoparticles, as well as, addressed the problem of air pollution by developing particle capture technologies for ‘bad’ nanoparticles. For material synthesis at industrial scale, flame aerosol reactors are extensively used for large-scale industrial production of ‘good’ nanoparticles. But, there exists a knowledge gap in understanding the early stages (1-10 nm) of particle formation and growth, which is necessary for tailoring the synthesized nanoparticles’ properties. To achieve this goal, measurement tools for the characterization of 1-10 nm particles are quintessential. On the other hand, to capture ‘bad’ particles, existing control technologies perform fairly well for particles > 200 nm, but fail to capture smaller nanoparticles. My dissertation is divided into three sections. First section focuses on developing the understanding of early stages of particle formation and growth (1-10 nm) during material synthesis in flame reactors both through experiments and modeling. Second section focuses on developing measurement tools for precise measurement of 1-10 nm sized particles, both through measurements, and simulations. Third section focuses on investigating plasma reactors as a possible approach for particle capture in conjunction with electrostatic precipitators. Part 1a: In this part, spherical, neutral particle-particle interaction is studied, and collisional growth rate is calculated both through controlled furnace experiments, as well as, molecular dynamic simulations. Controlled experiments are performed at moderately high temperatures from 400 – 800 C; and it was found that the collisional growth rate is higher than predicted by kinetic theory of gases. Experiments show that there is higher enhancement in the collisional growth as compared to theories based on Hamaker constant. Following this, molecular dynamics simulations are performed to understand the interatomic forces that lead to this enhancement in collisional growth using LAMMPS. . Part 1b: After focusing on particle-particle interactions in Part 1a, particle-ion interactions are studied both through experiments, and population balance modeling. Different factors like the effect of diluent in the flame (N2 v/s Ar), sampling height, and precursor concentration is evaluated on the percentage of charged particles. It is found that increasing the precursor concentration, and decreasing the sampling height leads to higher fraction of charged particles. To further understand the role played by charge, a population balance model for simultaneous charging and coagulation is developed using the method of moments. Part 2: The measurement tools for the classification and counting of nanoparticles are developed in this section. For classification, the new half-mini differential mobility analyzer is set-up in the lab, with high voltage source, and blower with high flow rates (100-600 lpm), and calibrated using electrospray set-up. This instrument is then characterized using commercial software COMSOL Multiphysics, and the simulated transfer function and resolution is compared with existing models, and experiments. This work provides a useful method to study the flow regimes and transfer function of a high flow DMA. On the other hand, for counting of nanoparticles less than 3 nm in size, the detection efficiency of conventionally used instrument condensational particle counter (CPC) is enhanced, and characterized for different composition of sub-3 nm particles. Part 3: This part of the dissertation will discuss about the fundamental understanding of particle-ion interactions in a non-thermal plasma reactor, with a vision to incorporate plasma reactors in conjunction with the conventionally used electrostatic precipitators, thereby increasing their efficiency for particle capture. Premade, charge-neutral nanoparticles are introduced into the plasma. The charge fraction and distribution of the particles are examined at the reactor outlet for different mobility diameters (10-250 nm) as a function of plasma power and two different types of power sources, alternating current (AC) and radio-frequency (RF). We find that the overall charge fraction increases with increasing plasma power and diameter for the RF plasma. A similar increasing trend is observed for the AC plasma with increasing particle diameter in the range of 50-250 nm, but the charge fraction increased with decreasing particle diameter in the range of 10-50 nm. The charge distribution is revealed to be bipolar with particles supporting multiple charges for both the RF and AC plasmas. Differences in the characteristic time scales for particle charging in the AC and RF plasmas are discussed which provide a possible explanation for the trends observed in the experiments.
Michel Attoui, Richard Axelbaum, Peng Bai, Rajan Chakrabarty,