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.

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    A Review of Metastable Beta Titanium Alloys
    (MDPI, 2018-06-30) Kolli, R. Prakash; Devaraj, Arun
    In this article, we provide a broad and extensive review of beta titanium alloys. Beta titanium alloys are an important class of alloys that have found use in demanding applications such as aircraft structures and engines, and orthopedic and orthodontic implants. Their high strength, good corrosion resistance, excellent biocompatibility, and ease of fabrication provide significant advantages compared to other high performance alloys. The body-centered cubic (bcc) β-phase is metastable at temperatures below the beta transus temperature, providing these alloys with a wide range of microstructures and mechanical properties through processing and heat treatment. One attribute important for biomedical applications is the ability to adjust the modulus of elasticity through alloying and altering phase volume fractions. Furthermore, since these alloys are metastable, they experience stress-induced transformations in response to deformation. The attributes of these alloys make them the subject of many recent studies. In addition, researchers are pursuing development of new metastable and near-beta Ti alloys for advanced applications. In this article, we review several important topics of these alloys including phase stability, development history, thermo-mechanical processing and heat treatment, and stress-induced transformations. In addition, we address recent developments in new alloys, phase stability, superelasticity, and additive manufacturing.
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    Fabrication and Characterization of Polypyrrole/Gold Bilayer Microactuators for Bio-Mems Applications
    (2005-08-25) Liu, Yingkai; Smela, Elisabeth; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    The proof of concept for conjugated polymer bilayer microactuators had been demonstrated prior to this dissertation with numerous devices, and their advantages in biomedical applications had been recognized. The next step for this technology was implementation in real systems, which required knowledge of the main performance metrics and limitations. In this dissertation, work focused on measuring these metrics for the first time to facilitate the development of cell-clinics, which are microsystems for cell study and for cell-based sensing. The conjugated polymer used throughout the dissertation was polypyrrole doped with dodecylbenzenesulfonate, PPy(DBS), and the second layer in the bilayer was gold. Device fabrication challenges were first identified and addressed, focusing particularly on methods to produce PPy/Au bilayers that did not suffer from delamination. By electroplating Au onto the electrodes or by wet etching them to increase mechanical interlocking, this problem, which had plagued the field for the last decade, was solved. Another important contributor to lifetime, which is a key actuator metric, is loss of electro-activity with extended cycling. This metric was quantified through measurements of the total exchanged charge of PPy(DBS) with cycles of electromechanical redox. This result impacts how these actuators can be used. Two other key metrics on which this work focused were bending angle, analogous to stroke in a linear actuator, and force. It was necessary to determine bending angle as a function of film thickness experimentally because the traditional bilayer beam models could not account for microfabricated bilayer radius of curvature data. Through experimental testing over a wide range of PPy and Au thicknesses, the relationship between PPy:Au thickness ratio and curvature was mapped out. The experimental results demonstrated the existence of strain gradients within the conjugated polymer films, with the material at the surface having greater actuation strain than that at the gold interface. Finally, accurate force measurements had not been done prior to this dissertation research because of the significant challenges involved in developing a method for measuring force in microactuators. This dissertation described the development of such a methodology and provides data for the blocked force as a function of polypyrrole thickness.