Theses and Dissertations from UMD

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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.

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    ATOMIC LAYER DEPOSITION OF LEAD ZIRCONATE-TITANATE AND OTHER LEAD-BASED PEROVSKITES
    (2019) Strnad, Nicholas Anthony; Phaneuf, Raymond J; Polcawich, Ronald G; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    Lead-based perovskites, especially lead zirconate-titanate (PbZrxTi1-xO3, or PZT), have been of great technological interest since they were discovered in the early 1950s to exhibit large electronic polarization. Atomic layer deposition (ALD) is a thin-film growth technique capable of uniformly coating high aspect-ratio structures due to the self-limited nature of the precursor chemisorption steps in the deposition sequence. In this thesis, a suite of related processes to grow lead-based perovskites by ALD are presented. First, a new process to grow ferroelectric lead titanate (PbTiO3, or PTO) by ALD using lead bis(3-N,N-dimethyl-2-methyl-2-propanoxide) [Pb(DMAMP)2] and tetrakis dimethylamino titanium [TDMAT] as the lead and titanium cation precursors, respectively, is discussed. A 360-nm thick PTO film grown by ALD displayed a maximum polarization of 48 µC/cm2 and remanent polarization of ±30 µC/cm2. Second, a new process (similar to the ALD PTO process) to grow PZT by ALD is demonstrated by partial substitution of TDMAT with either tetrakis dimethylamino zirconium or zirconium tert-butoxide. The 200 nm-thick ALD PZT films exhibited a maximum polarization of 50 µC/cm2 and zero-field dielectric constant of 545 with leakage current density < 0.7 µA/cm2. Third, a new ALD process for antiferroelectric lead hafnate (PbHfO3, or PHO) is presented along with electrical characterization showing a field-induced antiferroelectric to ferroelectric phase transition with applications for capacitive energy storage. Finally, ALD lead hafnate-titanate (PbHfxTi1-xO3, or PHT), considered to be an isomorph of PZT, is demonstrated by combining the process for PTO and PHO. The thin-film PHT grown by ALD is shown to have electronic properties that rival PZT grown at compositions near the morphotropic phase boundary (MPB). The processes for both ALD PZT and PHT are shown to yield films with promising properties for microelectromechanical systems (MEMS) actuators and may help to dramatically increase the areal work density of such devices.
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    COMBINATORIAL DISCOVERY OF A MORPHOTROPIC PHASE BOUNDARY IN A LEAD-FREE PIEZOELECTRIC MATERIAL
    (2008-08-07) Fujino, Shigehiro; Takeuchi, Ichiro; Wuttig, Manfred; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    BiFeO3 (BFO) is known to display rich and intricate multiferroic and chemical properties. It has high switchable polarization and piezoelectric properties similar to PbTiO3, but it also suffers from high coercive fields and high leakage currents. This has prompted investigations of doped-BFO as novel solid solutions which may emulate the performance of PbZr1-xTixO3 (PZT) in the composition range of 0.48 < x < 0.52, where a morphotropic phase boundary (MPB) resides and displays substantially enhanced piezoelectric and ferroelectric properties. However neither an increase in electromechanical constants as a function of dopant concentration or domain structures indicative of a MPB piezoceramic had been reported in lead-free BFO-based systems prior to this work. There are some guidelines which predict the presence of MPBs, and one can explore novel compositions by systematically searching for similar structural transitions, Yet comprehensive mapping of compositions requires synthesis of an enormously large number of individual samples. We report on the discovery of a lead-free morphotropic (composition dependent, temperature independent) phase boundary with a simple perovskite structure. The combinatorial thin film strategy was used to identify a rhombohedral to pseudo-orthorhombic structural transition　which exhibits a ferroelectric to antiferroelectric transition at approximately Bi0.86Sm0.14FeO3. At the morphtropic phase boundary, there is substantial enhancement in the dielectric and piezoelectric coefficient of Bi0.86Sm0.14FeO3 film is comparable to that of Pb Zr0.52Ti0.48O3 thin film. The discovered compound is a strong candidate if a lead-free piezoelectric material.