Theses and Dissertations from UMD
Permanent URI for this communityhttp://hdl.handle.net/1903/2
New submissions to the thesis/dissertation collections are added automatically as they are received from the Graduate School. Currently, the Graduate School deposits all theses and dissertations from a given semester after the official graduation date. This means that there may be up to a 4 month delay in the appearance of a give thesis/dissertation in DRUM
More information is available at Theses and Dissertations at University of Maryland Libraries.
Browse
2 results
Search Results
Item Fabrication and Characterization of Nanoscale Shape Memory Alloy MEMS Actuators(2020) Knick, Cory R.; Bruck, Hugh; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The miniaturization of engineering devices has created interest in new actuation methods capable of large displacements and high frequency responses. Shape memory alloy (SMA) thin films have exhibited one of the highest power densities of any material used in these actuation schemes with thermally recovery strains of up to 10%. With the use of a biasing force, such as from a passive layer in a “bimorph” structure, homogenous SMA films can experience reversible shape memory effect provided they are thick enough that the crystal structure is capable of transforming. However, thick films exhibit lower actuation displacements and speeds because of the larger inertial resistance. Therefore, there is a need to find a way to process thinner SMA films with grain structures that are capable of transformation in order to realize larger actuation displacements at higher speeds. In this work, a near-equiatomic NiTi magnetron co-sputtering process was developed to create nanoscale thick SMA films as thin as 120 nm. By using a metallic seed layer, it was possible to induce the crystallization of epitaxial, columnar grains exhibiting the shape memory effects in nanoscale films ranging from 120 – 400 nm. It was also possible to crystalize these SMA films at lower processing temperatures (as low as 325 °C) compared to directly sputtering thicker films onto Si wafers. The transformation behavior associated with the SME in these films were characterized using x-ray diffraction (XRD), differential scanning calorimetry (DSC), and stress-temperature measurements at wafer level. After quantifying the shape memory effects at wafer-level, the SMA films were used to fabricate various microscale MEMS actuators. The SMA films were mated in several “bimorph” configurations to induce out of plane curvature in the low-temperature Martensite phase. The curvature radius vs. temperature was characterized on MEMS cantilever structures to elucidate a relationship between residual stress, recovery stress, radius of curvature, and degree of unfolding. SMA MEMS actuators were fabricated and tested using joule heating to demonstrate rapid electrical actuation of NiTi MEMS devices at some of the lowest powers (5-15 mW) and operating frequencies (1-3 kHz) ever reported for SMA or thermal actuators. By developing a process to create nanoscale thickness NiTi SMA film, we enabled the fabrication of MEMS devices with full, reversible, actuation as low as 0.5 V. This indicated the potential of these devices to be used for high frequency, low power, and large displacement applications in power constrained environments (i.e. on chip).Item Functional imaging of photovoltaic materials at the nanoscale(2018) Tennyson, Elizabeth; Leite, Marina S; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The ideal photovoltaic technology for global deployment must exhibit two key attributes: (i) high power-conversion efficiency, enabling a solar panel with a large power output per area and, (ii) low-cost/W, due to either being derived from earth-abundant materials and/or ease of fabrication. For the past two decades, extensive efforts have been made to boost the efficiency of some of the most promising high performance and low-cost photovoltaic materials, such as CdTe, Cu(In,Ga)Se2 (CIGS), and hybrid organic-inorganic perovskites, to achieve higher efficiency devices. However, improvement in the overall performance is still limited by the open-circuit voltage (Voc). All of the solar cell materials listed above are composed of grains and grain boundaries on the order of micro- and nanometers, respectively, and their nanoscale interfaces can cause electrical charge carriers to become trapped and recombine non-radiatively, reducing the Voc. Therefore, in this thesis, I implement high spatial resolution functional imaging techniques to resolve the local voltage variations in the thin-lm polycrystalline and hybrid perovskites materials for photovoltaic applications. First, I spectrally and spatially resolve the local photovoltage of CIGS solar cells through confocal optical microscopy to build a qualitative voltage tomography. From these photovoltage results, I discover variations in the electrical response of >20% that are also on the same length scale as the grains composing the CIGS material. Therefore, by enhancing the spatial resolution beyond the diffraction limit, the electronic properties of individual grains and the interfaces between the grains can be fully resolved. For this, I implement Kelvin probe force microscopy (KPFM), and demonstrate a universal method to directly map the Voc of any photovoltaic material with nanoscale spatial resolution. Next, we extend this ability of KPFM to rapidly image (16 sec/map) the real-time dynamics of perovskite solar cells, which are notorious for their slow and unstable electrical output. Through fast-KPFM imaging, we discover regions within a single grain that show a residual Voc response which pervades for ~9 min, likely caused by a slow ion migration process. Finally, to understand how dierent perovskite compositions influence the behavior of the nanoscale electrical response, I utilize KPFM to realize both irreversible and reversible Voc signals. Compiling all these results discussed above, throughout my Ph.D. I have yielded the following contributions: (i) evidence that the photovoltage of polycrystalline solar cell materials varies at the same length scale as the grains composing them, (ii) a nanoscale imaging platform to directly map the Voc with unprecedented spatial resolution, and (iii) a technique to map the real-time voltage response of many perovskite compositions, ultimately indicating that the elements constituting the perovskite cation and halide positions are both directly related to their reversible vs. irreversible electrical nature. From these contributions, I foresee the functional imaging methods developed in this thesis to be widely implemented as a diagnostic tool for the rational design of photovoltaics with enhanced electrical performance and lower cost.