Browsing by Author "Doshi, Sagar M."

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    Adhesion Characterization and Enhancement between Polyimide-Silica Composite and Nodulated Copper for Applications in Next-Generation Microelectronics
    (ACS Applied Materials & Interfaces, 2024-01-17) Doshi, Sagar M.; Barry, Alexander; Schneider, Alexander; Parambil, Nithin; Mulzer, Catherine; Yahyazadehfar, Mobin; Samadi-Dooki, Aref; Foltz, Benjamin; Warrington, Keith; Wessel, Richard; Zhang, Lei; Simone, Christopher; Blackman, Gregory S.; Lamontia, Mark A.; Gillespie, John W. Jr.
    As the need for high-speed electronics continues to rise rapidly, printed wiring board (PWB) requirements become ever-more demanding. A typical PWB is fabricated by bonding dielectric films such as polyimide to electrically conductive copper foil such as rolled annealed (RA) copper and is expected to become thinner, flexible, durable, and compatible with high-frequency 5G performance. Polyimide films inherently feature a higher coefficient of thermal expansion (CTE) than copper foils; this mismatch causes residual thermal stresses. To attenuate the mismatch, silica nanoparticles may be used to reduce the CTE of PI. A nodulated copper surface can be used to enhance the Cu/PI adhesion by additional bonding mechanisms that could include a type of mechanical bonding, which is a focus of this study. In this investigation, a 90° peel test was used to measure the peel strength in copper/polyimide/copper laminates containing nodulated copper and polyimide reinforced with 0, 20, and 40 wt % silica nanoparticles. The influence of silica nanoparticles on the peel strength was quantitatively evaluated. Laminates incorporating polyimide films lacking silica nanoparticles had a ∼3.75× higher peel strength compared with laminates reinforced with 40% silica. Their failure surfaces were analyzed by using scanning electron microscopy (SEM), energy-dispersive X-ray analysis (EDX), and X-ray photoelectron spectroscopy to identify the mode of failure and to understand bonding mechanisms. The key bonding mechanism, mechanical interlocking, was achieved when the polyimide surrounded or engulfed the copper nodules when the laminate was created. Post-testing failure surface analysis revealed the presence of copper on the polyimide side and polyimide on the copper side, indicating mixed mode failure. An analytical model was developed to determine the impact of applied pressure, temperature, and time on the polyimide penetration and mechanical interlocking around the copper nodules. The model was validated by measuring the peel strength on another set of specimens fabricated using increased temperature and pressure that showed a 3× increase in peel strength compared to lower temperature/pressure processing conditions. This enhanced adhesion resulted from the lower polymer material viscosity at higher temperatures, which fosters deeper and more complete penetration around the copper nodules during processing at higher pressures for longer durations. The methodology of combining peel testing, viscosity and CTE measurement, SEM/EDX, surface chemical analysis, and penetration depth calculation developed herein enables the calculation of the desired processing parameters to enhance functionality and improve adhesion.
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    Processing, characterization and applications of carbon nanotube based hierarchical composites for structural health monitoring and wearable sensors
    (University of Delaware, 2020) Doshi, Sagar M.
    Advances in the processing science and characterization tools for nanomaterials have stimulated tremendous research for using carbon nanotubes in various innovative applications. The exceptionally high specific strength and stiffness, remarkable electrical and thermal properties along with high-aspect ratios of carbon nanotubes make them an ideal candidate for their use in hierarchical multiscale composites as well as nanocomposite films. This research is a combination of fundamental research to understand processing/properties relationships and applied science for developing novel multifunctional applications using carbon nanotubes. ☐ The overarching objective of this dissertation is to advance the basic knowledge in using electrophoretic deposition to create conductive nanocomposite films of carbon nanotubes on non-conductive fabrics and seek to understand the fundamental sensing mechanisms. The multifunctional performance of both carbon nanotube-based hierarchical composites and nanocomposite coated textiles is evaluated for potential applications in structural health monitoring and wearable sensors. ☐ Aqueous electrophoretic deposition has immense potential to be scaled up for industrial production, but fundamental gaps in knowledge remain to be bridged. In this research, the film formation mechanism of functionalized carbon nanotubes on non-conductive fabrics is investigated by conducting experiments at multi-length scales, from fiber bundles to macro composites using novel characterization techniques. ☐ Leveraging electrophoretic deposition and other scalable processing techniques, applications in structural health monitoring, and wearable sensors are developed, tested, and validated. The strain sensing response of carbon nanotube-based sensing skins fabricated using different processing techniques is investigated and evaluated. A comprehensive characterization of the sensing skins under different loading conditions is performed. A novel methodology for calculation of the gage factor, which is independent of the substrate material properties is established through innovative tests using a biaxial testing machine. The carbon nanotube-based sensor is also used for damage sensing in adhesively bonded hybrid metal and composite joints. The ability to detect incipient damage and distinguish between different failure modes is demonstrated. ☐ Other applications explored include flexible pressure sensors with an extremely wide range of pressure detection and flexible garment based stretch sensors for application in human joint motion analysis. The flexible pressure sensors display the capability to detect an ultrawide range of pressure, from touch to tons. In collaboration with biomechanics researchers, the sensors are integrated with footwear and validated in a gait laboratory for applications in human gait analysis. The garment based stretch sensors display a remarkable sensitivity. A resistance change of 3,000% is observed when the sensor is worn over the elbow/knee. Due to the extremely thin nanocomposite coating, the texture of the fabric does not change significantly, making the stretch sensors breathable and comfortable to wear. These novel wearable sensors made using a scalable process have the potential to stimulate research and development in the fields of human-computer interfacing, gesture recognition, and monitoring the rehabilitation process after an injury.