Theses and Dissertations from UMD
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Item Formation and Characterization of Transversely Modulated Nanostructures in Metallic Thin Films using Epitaxial Control(2013) Boyerinas, Brad Michael; Bruck, Hugh A; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This thesis describes a fundamental investigation into the formation, characterization, and modeling of epitaxially-controlled self-assembly at the nanoscale. The presence of coherent nanophases and the clamping effect from an epitaxial substrate enables the formation of transversely modulated nanostructures (TMNS) resulting in improved functionality, which was previously observed through increased piezoelectric response in BiFeO3. The ability to fabricate high quality epitaxial films presents opportunity to investigate coherent phase decomposition in other material systems with multifunctional response. The research herein aims to extend the concept of nanoscale self assembly in metallic systems, including Ag-Si and Pd-PdH. First, the effect of annealing a Ag-Si couple was examined, and ordered, nanoscale Ag crystallites were observed along the interface with the epitaxial Si wafer. It is demonstrated that Ag foil can be used in place of doped Ag paste (commonly used in solar cell metallization) to achieve TMNS at the interface. It was proved that annealing the Ag-Si couple in air is necessary for the self-assembly reaction to take place, as doing so prevents bulk diffusion and eutectic melting. Electron backscatter diffraction was used to verify the epitaxial relation between the Ag nanostructures and Si crystal. A method to fabricate ordered, nanoscale PdH precipitates in epitaxial Pd thin films via high temperate gas phase hydrogenation was established. Epitaxial Pd films were deposited via e-beam deposition and a V buffer layer was necessary to induce epitaxy. This novel self-assembled nanostructure may enable hysteresis-less absorption and desorption, thus improving functionality with regard to hydrogen sensing and storage. The epitaxial Pd film was characterized before and after hydrogenation with x-ray diffraction and atomic force microscopy to determine composition and nanostructure of the film. A thermodynamic model was developed to demonstrate the possibility to control or eliminate thermodynamic hysteresis via balance of elastic interaction between the coherent interfaces of metal and metal-hydride phases and the film-substrate interface. This model can be extended to other metal-hydride systems which demonstrate coherent phase decomposition.Item ORIENTATION DEPENDENCE OF THE PIEZOELECTRIC PROPERTIES OF EPITAXIAL FERROELECTRIC THIN FILMS(2005-07-08) Ouyang, Jun; Roytburd, Alexander L.; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)There are both intrinsic piezoelectric response and extrinsic piezoelectric response in ferroelectric materials. The intrinsic piezoelectric response is due to the lattice deformation of a single-domain crystal, which can be characterized by tensors of piezoelectric constants. The extrinsic piezoelectric response depends on extrinsic sources of displacement under the electric field, which can be the movement of domain walls, phase boundaries, or even defects like grain boundaries or dislocations. Due to the elastic interaction between an epitaxial ferroelectric thin film and a substrate, the piezoelectric properties of an epitaxial ferroelectric film are different from those of bulk ferroelectric materials. This work is the first study on the general orientation dependence of the piezoelectric properties of epitaxial ferroelectric thin films, which includes both theoretical and experiment work on intrinsic and extrinsic piezoelectric properties of epitaxial ferroelectric films. A complete theoretical analysis of intrinsic piezoelectric responses in a single domain ferroelectric film, which are characterized by effective longitudinal,transverse and shear piezoelectric coefficients, is presented in this dissertation. On the part of extrinsic piezoelectric response, our recent work on the piezoelectric properties of epitaxial thick lead titanate zirconate (Pb(ZrxTi1-x)O3 with x=0.52) films with tetragonal distorted structures will be presented as an example. It is shown that(011) oriented epitaxial films had much enhanced piezoelectric responses as compared with those of (001) and (111) oriented films. Detailed structure analysis showed that instead of an interconnected 3-domain (3-D) architecture that is usually found in a (001) oriented thick film, the (011) films consisted of a dominant 2-domain (2-D) architecture, by which the pinning between neighboring domain walls is much reduced. This study demonstrate the possibility of achieving high extrinsic piezoelectric responses by optimizing the epitaxial relationship between the film and substrate with respect to the domain mobility, and should also be instructive to the design of ferromagnetic and ferroelastic thin film devices used for transducer applications.