Mechanical and Aerospace Engineering electronic theses and dissertations (MU)
https://hdl.handle.net/10355/5297
The electronic theses and dissertations of the Department of Mechanical and Aerospace Engineering within the College of Engineering at the University of Missouri-Columbia.2024-03-28T16:23:19Z3D growth simulation and field emission properties of vertically aligned carbon nanotubes
https://hdl.handle.net/10355/98799
3D growth simulation and field emission properties of vertically aligned carbon nanotubes
Bellott, Elizabeth
[EMBARGOED UNTIL 12/1/2024] Since their discovery, carbon nanotubes (CNTs) have become a widely researched and utilized nanomaterial due to their nanoscale size and unique electrical, thermal, and mechanical properties. In particular, bulk structures of aligned CNTs, such as CNT forests, exhibit nanoscale-based properties desirable for various electrical applications. Unfortunately, the complexity of CNT systems makes the properties of CNT forests difficult to predict from synthesis conditions. Chapter 2 details the experimental investigation and 2D simulation of the piezoresistive response of CNT-coated microfibers as hairlike sensors. Electromechanical tests reveal that piezoresistivity is governed by contact resistance between measurement electrodes and the free ends of CNT 'hairs.' Hairlike sensors must be highly sensitive to external stimuli, indicating the need for a large, consistent stimulus response. Results suggest that small diameter microfibers with short CNT forests provide the highest piezoresistive sensitivity, supporting the excellent potential of CNT- coated microfibers as artificial hairlike sensors. Chapter 3 introduces a time-resolved 3D CNT forest growth simulation to serve as a diagnostic tool for designing and predicting the properties of experimental CNT forests. 3D simulated CNT forest number density and conductance results are compared against CNT forest properties determined by in situ scanning electron microscopy (SEM) growth experiments and post-growth small angle x-ray spectroscopy (SAXS) measurements. Preliminary results establish that with additional adjustments, the 3D CNT growth simulation model can provide excellent representations of CNT forest growth. Chapter 4 concerns the initial fabrication, characterization, and field emission testing of CNT-based field emission cathodes for high-power physics applications, specifically a custom-designed klystron called a 'klystrino.' Both high density forest (HDF) and patterned CNT cathodes are fabricated by fixed catalyst chemical vapor deposition (CVD). Field emission tests upon CNT HDF cathodes and CNT micropillar cathodes confirm characteristic Fowler-Nordheim emission behavior and yield emission current densities on the order of A/m2.
2023-01-01T00:00:00Z3D polymeric scaffolds towards biomedical applications
https://hdl.handle.net/10355/78016
3D polymeric scaffolds towards biomedical applications
Zhang, Cheng
[ACCESS RESTRICTED TO THE UNIVERSITY OF MISSOURI--COLUMBIA AT REQUEST OF AUTHOR.] How can research on mechanical engineering and materials science contribute to human health? The fabrication of biomedical scaffolds could be a good entry point. Scaffolds are broadly applied in biomedical fields with multiple functions, such as repair, replacement, and stimulation and monitoring when they are integrated with electronic/optoelectronic devices. Besides biocompatible, the scaffolds should be soft and in form of three-dimensional (3D) structures in order to mechanically and geometrically match the natural tissues and organs. Polymers are the most promising candidate materials for the scaffold fabrication. Compared to metals and ceramics, substantial polymers have biocompatibility and all of them have low Young's modulus and high processability. Benefiting from the high processability, a variety of approaches can be used to shape polymeric scaffolds with 3D architectures. The major three approaches are flexibility, stress induced assembly, and printing. However, none of them is flawless: (1) For flexibility, the scaffolds that integrated with electronic devices have large thickness which exponentially lower the flexibility. (2) For stress-induced assembly, the assembly operation requires complicated actuation equipment and the assembled scaffolds are usually tethered on cumbersome elastomeric substrates. (3) For printing, few of scaffolds fabricated by emerging 4D printing technologies are responsive to biocompatible stimuli. This dissertation aims at addressing these three problems. First, a new device structure, i.e., lateral electrode, is proposed to reduce the thickness and then improve the flexibility of the scaffolds with electronics, which is validated by fabricating flexible photodetectors on polyimide substrates. The photodetectors have excellent flexibility and can be bent to 3D structures. Second, a new stress-induced assembly strategy, i.e., responsive buckling, is developed in which the elastomeric substrates are replaced with deft responsive polymeric substrates. Various 3D polymeric scaffolds either with or without electronic devices are assembled when the substrates are exposed to external stimuli without manual intervention. This strategy is first verified by an acetone responsive organogel and then developed toward biomedical applications by using a body temperature responsive hydrogel. Third, a new shape memory polymer, i.e., poly (glycerol dodecanoate) acrylate (PGDA), whose transition temperature is in the range of 20-37 [degrees]C, is exploited for 4D printing of scaffolds. Because of the propriate transition temperature, the shape memory process of the scaffolds can be completed by using room temperature and body temperature as stimuli, which are harmless for human body. Moreover, a variety of delicate 3D structures including an artery-like tube are printed.
2020-01-01T00:00:00Z3D printing of active polymeric materials
https://hdl.handle.net/10355/70044
3D printing of active polymeric materials
Su, Jheng-Wun
[ACCESS RESTRICTED TO THE UNIVERSITY OF MISSOURI AT REQUEST OF AUTHOR.] Learning from nature livings, especially those that can respond to the stimuli and change the shape, is attracting increasing interests in a wide variety of research fields. There is a significant need of developing synthetic materials that can mimic these living systems to show dynamic and adaptive shape-changing functions. Although various fabrication methods including molding, micro-fabrication and photolithography have been developed to fabricate the dynamic materials, they all have shown some limits. At present, 3D printing is a promising technique, which provides a cost effective, accurate and customized method to form 3D structures. The recently new emerging technique, 4D printing, which employs the 3D printing to print the active materials for dynamic 3D structures, shows a great potential for various applications such as tissue engineering, flexible electronics, and soft robotics. Despite much recent progress, this technology and its application in 3D dynamic structure fabrication is still in its infancy. My Ph.D. dissertation focuses on 4D printing of programmable polymeric materials that exhibits complex, reversible, shape transformations as well as enriching the printable material library by exploring various active materials for 4D printing technology. Chapter 1 introduces the current development of active materials and methodologies. Much attention is paid to the recent progress and its merits and demerits. Chapter 2 presents a simple and inexpensive 4D printing of waterborne polyurethane paint (PU) composites that are fabricated by mixing PU with micro-size preswollen carboxymethyl cellulose (CMC) and silicon oxide nanoparticle (NPs), respectively. Chapter 3 presents the 4D printing of a commercial polymer, SU-8, which has yet been reported in this field. The self-morphing behaviors of the printed SU-8 structures are induced by spatial control of swelling medium inside the SU-8 matrix. In Chapter 4, machine learning algorithms are applied to evaluate the shape-morphing behaviors of 4D printed objects. After the model optimization by tuning the hyperparameters the obtained machine learning models enable to accurately predict the final curvatures and curving angles of the 4D printed SU-8 structures from given input geometrical information. This initial success show that these data-driven surrogate models can well circumvent the challenge of human centered trial-and-error process in optimizing the printed structures, thereby pushing the research in 4D printing to a new height.
2019-01-01T00:00:00Z3D printing of energetic material
https://hdl.handle.net/10355/70135
3D printing of energetic material
Countryman, Andrew Michael
In the world of macro-scale energetics, the problems of cost effectiveness, scalability, and manufacturability are of prime importance. But with the advent of 3D printing, a solution for scalability is in reach. In this work, THV and GO are explored as candidate materials for printing. To avoid subjectivity, the materials have to pass a test of repeatability, control of geometry, and layer building. Moreover, a novel drop on demand 3D printing system was explored as the printing method. The overall goal is to take the material out of the lab room and into a scalable manufacturing technique. After initial testing, it was decided that THV is suitable for 3D printing while GO needs to be developed further to enable printability.
2019-01-01T00:00:00Z