Unipolar Diffusion Charging of Spherical and Agglomerated Nanoparticles and its Application toward Surface-area Measurement

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Unipolar Diffusion Charging of Spherical and Agglomerated Nanoparticles and its Application toward Surface-area Measurement

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This thesis consists of two parts: unipolar diffusion charging of nanoparticles and its application on the method development of surface-area measurements. The electrical capacitance of aerosol particles indicates their potential diffusion charging level, which is important for their classification by electrical mobility, precipitation (removal or collection) in electrical fields, and morphology characterization. A minimum potential energy method was used to calculate the electrical capacitance for agglomerates composed of equally sized spherical primary particles (PPs). By discretizing the particle surface using finite spherical elements, as net charge only resides on the surface of an isolated conductor, this method was extended to calculate the capacitance of arbitrarily shaped particles. Based on the capacitance, the charge of these particles was obtained by diffusion charging theory. In addition, the dynamics of capacitance and mean charge of agglomerate during sintering or coalescence (at constant particle volume) to aggregates and finally to compact structures was computed and found in agreement with sparse experimental data. Particle morphology strongly affects the capacitance and mean charge of fractal-like particles. For example, both decreased by 60% upon full coalescence or sintering of an agglomerate consisting initially of 128 PPs. Although geometric surface area (GSA) of nanoparticles has received much attention in many fields (drug delivery, catalysts, inhalation exposure, toxicity, etc.), no appropriate instruments and methods for online measurements of GSA are readily available. Therefore, this study intends to develop a Geometric Surface Area Monitor (GSAM) to measure the GSA of spherical as well as model agglomerate/aggregate nanoparticles in nearly real-time. The GSAM has two versions: 1. The GSAM (I) consists of several existing techniques in series, including inertial impaction, unipolar charging, electrostatic precipitation, and electrical current measurement. The GSAM (I) was first evaluated and calibrated by measuring the GSA of monodisperse nanoparticles. Spherical, aggregate, and agglomerate nanoparticles were tested in the calibration. It was found that the measured electrical current was proportional to the surface area concentration. The calibration curves obtained from the measurements of monodisperse particles was then applied for polydisperse spherical particles and compared the measured GSA with that determined by the well-known scanning mobility particle sizer (SMPS) where the GSAM (I) had less than 10% of deviation compared with SMPS. 2. In the GSAM (II), the commercialized nanoparticle surface area monitor was used and slightly modified. The instrument responses under two different conditions were combined in a weighted sum (WS) fashion to correlate with the aerosol GSA concentration. We present the GSA concentration results and comparisons with well-known SMPS data in both laboratory testing and field measurement. For the laboratory testing, the two methods have a good agreement with a Pearson correlation coefficient of 0.9961; for the field measurements including the indoor and outdoor samplings, both methods agree well with each other. In addition, the new WS method is more stable in the clean indoor air and suitable for outdoor environmental sampling with a slight overestimation (125% of SMPS). These three studies below comprise parts of the main body of this dissertation and have been published. Chapter 2: Cao, L. N. Y., Wang, J., Fissan, H., Pratsinis, S. E., Eggersdorfer, M. L., & Pui, D. Y. H. (2015). The capacitance and charge of agglomerated nanoparticles during sintering. Journal of Aerosol Science, 83(0), 1-11. doi: http://dx.doi.org/10.1016/j.jaerosci.2015.01.002 Chapter 3: Cao, L. N. Y., Chen, S.-C., Fissan, H., Asbach, C., & Pui, D. Y. H. (2017). Development of a geometric surface area monitor (GSAM) for aerosol nanoparticles. Journal of Aerosol Science, 114, 118-129. doi: https://doi.org/10.1016/j.jaerosci.2017.09.013 Chapter 4: Cao, L. N. Y. & Pui, D. Y. H. A novel weighted sum method to measure particle geometric surface area in real-time, Journal of Aerosol Science, doi: https://doi.org/10.1016/j.jaerosci.2017.12.007


University of Minnesota Ph.D. dissertation. December 2017. Major: Mechanical Engineering. Advisor: David Pui. 1 computer file (PDF); xi, 123 pages.

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Cao, Nanying. (2017). Unipolar Diffusion Charging of Spherical and Agglomerated Nanoparticles and its Application toward Surface-area Measurement. Retrieved from the University Digital Conservancy, https://hdl.handle.net/11299/194630.

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