Mechanical Engineering

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    Direct visualization of nanoparticle morphology in thermally sintered nanoparticle ink traces and the relationship among nanoparticle morphology, incomplete polymer removal, and trace conductivity
    (Institute of Physics, 2023-06-19) Chandel, Ghansham Rajendrasingh; Sun, Jiayue; Etha, Sai Ankit; Zhao, Beihan; Sivasankar, Vishal Sankar; Nikfarjam, Shakiba; Wang, Mei; Hines, Daniel R.; Dasgupta, Abhijit; Woehl, Taylor; Das, Siddartha
    A key challenge encountered by printed electronics is that the conductivity of sintered metal nanoparticle (NP) traces is always several times smaller than the bulk metal conductivity. Identifying the relative roles of the voids and the residual polymers on NP surfaces in sintered NP traces, in determining such reduced conductivity, is essential. In this paper, we employ a combination of electron microscopy imaging and detailed simulations to quantify the relative roles of such voids and residual polymers in the conductivity of sintered traces of a commercial (Novacentrix) silver nanoparticle-based ink. High resolution transmission electron microscopy imaging revealed details of the morphology of the inks before and after being sintered at 150 °C. Prior to sintering, NPs were randomly close packed into aggregates with nanometer thick polymer layers in the interstices. The 2D porosity in the aggregates prior to sintering was near 20%. After heating at 150 °C, NPs sintered together into dense aggregates (nanoaggregates or NAgs) with sizes ranging from 100 to 500 nm and the 2D porosity decreased to near 10%. Within the NAgs, the NPs were mostly connected via sintered metal bridges, while the outer surfaces of the NAgs were coated with a nanometer thick layer of polymer. Motivated by these experimental results, we developed a computational model for calculating the effective conductivity of the ink deposit represented by a prototypical NAg consisting of NPs connected by metallic bonds and having a polymer layer on its outer surface placed in a surrounding medium. The calculations reveal that a NAg that is 35%–40% covered by a nanometer thick polymeric layer has a similar conductivity compared to prior experimental measurements. The findings also demonstrate that the conductivity is less influenced by the polymer layer thickness or the absolute value of the NAg dimensions. Most importantly, we are able to infer that the reduced value of the conductivity of the sintered traces is less dependent on the void fraction and is primarily attributed to the incomplete removal of the polymeric material even after sintering.
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    Electrospray-Differential Mobility Analysis of Bionanoparticles
    (2012) Guha, Suvajyoti; Zachariah, Michael R; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    The growth of the multibillion dollar bionanoparticle industry has spurred the development of new physical characterization methods. One such method, electrospray-differential mobility analysis (ES-DMA) constitutes an electrospray for aerosolization of bionanoparticles (such as viruses, gold-nanoparticles, proteins, nanoparticle-protein complexes) and an ion mobility method that operates at atmospheric conditions, and separates bionanoparticles spatially. This dissertation identifies some relevant "problem" areas for ES-DMA by reviewing selected applications. Some such problems are: proteins while passing through ES capillaries are found to interact with it and thus produce time dependent size distributions. Further, it is thought that adsorbed proteins may subsequently desorb and influence size distributions with the ES-DMA which may concomitantly affect quantification of aggregates. These artifacts are studied systematically and it is demonstrated that ES-DMA can quantify adsorption-desorption of complex protein mixtures at high shear rates. Further, it is shown that desorbing proteins do not have a significant effect on size distributions. Another artifact of the ES takes place during the aersolization process. Two units (called monomers) of a bionanoparticle may get encapsulated in the same ES droplet and upon drying of the droplet create artificial dimers thus affecting quantification with ES-DMA. Assuming Poisson distribution, this thesis provides a systematic approach that can be undertaken to eliminate this artifact. A third artifact arises from the low sensitivity of the DMA to size increase. When a ligand (for e.g. protein) adsorbs to a bionanoparticle it creates an increase in the size of the later, which can be used to quantify the amount of ligand adsorbed per bionanoparticle. As ligands can change conformations upon adsorption, using ES-DMA for such applications may be flawed. This issue has been identified and a solution has been provided by integrating a mass analyzer after the ES-DMA. After correcting for these artifacts, this dissertation delves into characterization of different types of bionanoparticles and demonstrates that ES-DMA has several advantages over other traditional techniques such as transmission electron microscopy, size exclusion chromatography, analytical ultracentrifugation, dynamic light scattering and plaque assay and thus has immense potential to become a process analytical technique in biomanufacturing environments.