Mechanics and charging of nanoparticle agglomerates.
2009-06
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Mechanics and charging of nanoparticle agglomerates.
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2009-06
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Thesis or Dissertation
Abstract
This thesis consists of two parts. The first part concerns studies on mechanics of
real agglomerate particles and the second part involves studies on unipolar diffusion
charging of agglomerates.
Understanding mechanics of real agglomerate particles consisting of multiple
primary particles is important for aerosol sizing instrumentation using electrical
mobility and nanoparticle manufacturing process where coagulation and sedimentation
occur. A key quantity determining transport properties of agglomerates is the friction
coefficient. However, quantitative studies for the friction coefficient of agglomerates
are very limited. Transmission Electron Microscopy (TEM) image analysis results of
silver agglomerates provides a basis for the comparison of experimental data with
estimates based on free molecular models. A new quantitative method to determine the
dynamic shape factor and the two exponents, η and Dfm, which characterize the power
law dependence of friction coefficient on the number of primary spheres and the mass
on the mobility diameter, was developed using Differential Mobility Analyzer (DMA)-
Aerosol Particle Mass (APM) analyzer. Model predictions indicate that η is independent
of agglomerate size while Dfm is sensitive to agglomerate size. Experimentally, it
appears the opposite is true. Tandem DMA (TDMA) results also show that the massmobility
diameter scaling exponent is not dependent on mobility size range. Estimates
of non-ideal effects on the agglomerate dynamics were computed as perturbations to the Chan-Dahneke agglomerate model. After the corrections, an agreement between
experimental data and model predictions becomes significantly improved.
Unipolar diffusion charging becomes more attractive because it has higher
charging efficiency than bipolar charging as well as important applications in aerosol
sizing instrumentation using electrical mobility, powder coating, and the removal of
toxic particles from air stream using Electrostatic Precipitator. It has been reported that
the particle morphology affects both bipolar and unipolar charging processes.
Nevertheless, knowledge about the charging of non-spherical particles such as asbestos
fibers and fractal agglomerates is still lacking. From this study it was found that the
effect of dielectric constant of materials on unipolar diffusion charging of nanoparticles
is very small and the experimental results are in a good agreement with Fuchs (1963)’
theory. The effect of agglomerate morphology on unipolar charging characteristic was
examined both experimentally and analytically in terms of the mean charge per particle.
Both geometric surface area and electrical capacitance are known as two important
parameters to determine the mean charge of non-spherical particles. A new model to
predict the electrical capacitance of loose agglomerate particles as a function of
mobility diameter was developed incorporating electrical mobility and electrostatics theories. This study shows that the electrical capacitance contributes to increase the
mean charge per particle of agglomerates more than the geometric surface area,
especially in the transition regime. The estimates of geometric surface area and
electrical capacitance were used to predict the mean charge from Chang (1981)’s model
and the predicted results are reasonably in good agreement with experimental data.
Description
University of Minnesota Ph.D. dissertation. June 2009. Major: Mechanical Engineering. Advisor: Dr. David Y. H. Pui. 1 computer file (PDF); xxii, 223 pages, appendices A-B.
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Shin, Weon Gyu. (2009). Mechanics and charging of nanoparticle agglomerates.. Retrieved from the University Digital Conservancy, https://hdl.handle.net/11299/54826.
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