A. James Clark School of Engineering

Permanent URI for this communityhttp://hdl.handle.net/1903/1654

The collections in this community comprise faculty research works, as well as graduate theses and dissertations.

Browse

Author: search.filters.author.Zhao, Beihan

Search Results

Now showing 1 - 3 of 3
  • Thumbnail Image
    Item
    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.
  • Thumbnail Image
    Item
    Explorations of Carbon-Nanotube-Graphene-Oxide Inks: Printability, Radio-Frequency and Sensor Applications, and Reliability
    (2022) Zhao, Beihan; Das, Siddhartha SD; Dasgupta, Abhijit AD; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    Carbon-Nanotube (CNT) is a novel functional material with outstanding electrical and mechanical properties, with excellent potential for various kinds of industrial applications. Additive manufacturing or 3D printing of CNT-based materials or inks has been studied extensively, and it is vital to have a thorough understanding of the fluid mechanics and colloidal science of CNT-based inks for ensuring optimum printability and the desired functionality of such CNT-based materials.In this dissertation, a custom-developed syringe-printable CNT-GO ink (GO: Graphene Oxide) is introduced and the fluid mechanics and colloidal science of this ink as well as the different devices (e.g., temperature sensor, humidity sensor, and RF antenna) fabricated with this ink are studied. The following topics are discussed in this dissertation: (1) the application and printability (in terms of the appropriate fluid mechanics and colloidal science) of CNT-based inks; (2) development of temperature sensors with CNT-GO inks; (3) development of humidity sensors with CNT-GO inks; (4) development of RF patch antenna with CNT-GO inks; and (5) evaporation-driven size-dependent nano-microparticulate three-dimensional deposits (CNTs serve as one type of nanoparticle examined in this part of the study). In Chapter 1 of this dissertation, a literature review is conducted on the application of CNT-based inks and the fluid mechanics and colloidal science issues dictating the printability and performance of such CNT-based inks. The problem statement and overall research plan are also introduced in this chapter. In Chapter 2, the development of our custom CNT-GO ink is introduced. Detailed material selection and the mechanism of shape-dependent arrest of coffee-stain effect, which ensured that the printable ink led to uniform deposition, are discussed in this chapter. Temperature sensor prototypes printed with the CNT-GO inks are also presented in Chapter 2. From Chapter 3 to Chapter 5, the performances of our CNT-GO based flexible temperature sensor, humidity sensor, and patch antenna prototypes are discussed. The ink printability on flexible thin PET films is studied, and a straightforward ‘peel-and-stick’ approach to use the CNT-trace (or patch)-bearing PET films on surfaces of widely varying wettabilities and curvatures as different prototypes is introduced. Excellent temperature and humidity sensitivity of our CNT-GO based sensors are presented in Chapter 3 and Chapter 4, and the potential of this CNT-GO material for fabrication of ultra-wideband (UWB) patch antennas is discussed in Chapter 5. Furthermore, the stability and reliability of these printed CNT-GO-based prototypes are also explored. In previous Chapters, the printed CNT-GO patterns were cured by evaporation-mediated deposition on flat substrates (i.e., 2D deposition spanning in x and y directions). This motivated the extension of the physics to the 3rd dimension and probing of particle deposition on a 3D substrate and particle deposition in all x, y, and z directions. Therefore, in Chapter 6, we perform an experiment to demonstrate this kind of possibility using three kinds of micro-nanoparticle-laden water-based droplets (i.e. coffee particles, silver nanoparticles, and CNTs). These droplets were first deposited at the bottom of an un-cured PDMS film; these droplets were lighter than the PDMS and hence, they rose to the top of the PDMS where they could have either attained a Neuman like state or simply remained as an undeformed spherical drop with the top of the drop breaching the air-liquid-PDMS interface. The calculations based on air-water, water-PDMS, and air-PDMS surface tension values confirmed that the Neuman like state was not possible, and the droplets were likely to retain their undeformed shapes as they breached the air-PDMS interface. The timescale differences between the fast PDMS curing and the slower droplet evaporation, led to the formation of spherical shape cavities inside the PDMS after completion of the curing, and allowed evaporation-driven deposition to occur in all x, y, and z directions inside the cavity, with the exact nature of the deposition being dictated by the sizes of the particles (as confirmed by the experiments conducted with coffee particles, silver nanoparticles, and CNTs). Finally, in Chapter 7, the major contributions of this dissertation and proposed future studies related to this dissertation work are listed.
  • Thumbnail Image
    Item
    An Experimental Study of Heat Transfer Performance Enhancement by Applying Oscillating Flexible Structures
    (2018) Zhao, Beihan; Yang, Bao; Liu, Xinan; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    In this thesis, the effects of a flow-induced oscillating flexible structure on the convective heat transfer of a plate heat exchanger in a vertical rectangular wind tunnel were experimentally investigated. A variable speed fan was used to create an air flow with speed ranging from 9 m/s to 20 m/s. A flexible structure was placed in the front of the plate heat exchanger, which would oscillate in the air flow. Different shapes and dimensions of flexible structures were tested. The temperature of the plate was monitored, and the steady-state temperature was recorded for each experimental condition. The oscillating motions of the flexible structures were captured with a high-speed camera. Compared to the steady flow convection, the use of the oscillation flexible structure can enhance the mixing of the high-temperature boundary flow with the lower temperature flow and disrupt the boundary layer. The length and width of the rectangular structures were found to have large influence on the oscillating characteristics and the convective heat transfer enhancement. Dimensionless parameters including flow-induced oscillation frequency, f^*, and coverage ratio, A^*, were also studied in order to discover the relationship between them. Experimental results showed that the structures with a rectangular shape can most significantly improve the convective heat transfer among those various shapes used in the present study. The average heat transfer coefficient was improved from 113 [W/(m^2*K)] to 161 [W/(m^2*K)] when the inlet wind velocity was 17.2 m/s, and that specific rectangular structure had a length of 0.075 [m] and a width of 0.102 [m].In addition, highest heat transfer performance was found when 0.22≤f^*≤0.32, which could be used for possible further design optimizations.