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  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.
401

Template-based Ferromagnetic Nanowires and Nanotubes: Fabrication and Characterization

Wei, Zhiyuan 03 October 2013 (has links)
This dissertation describes experimental studies of the structures and properties, and their correlations in ferromagnetic nanowires and nanotubes fabricated using porous templates. Ferromagnetic Ni and Fe nanowires with diameters 30 ~ 250 nm were electroplated into the pores of anodic aluminum oxide membranes. The effects of nanowire diameter on structural and magnetic properties were investigated. The microstructures of these nanowires were studied using X-ray diffraction and selected-area electron diffraction measurements. The magnetic properties of the nanowires were investigated using magnetic hysteresis measurements and magnetic force microscopy. Additionally, ferromagnetic Ni-P nanotubes were fabricated using an electroless chemical deposition method. Structure and composition analyses were conducted using X-ray diffraction and energy-dispersive spectroscopy. The magnetic properties of the nanotube arrays and the electronic properties of individual nanotubes were studied. Hysteresis measurements revealed that the 250-nm diameter Ni nanowires had a poor squareness in their hysteresis loops, indicating the existence of multi-domain states. In comparison, the squareness in the hysteresis loops of 60-nm and 30-nm Ni nanowires was much improved, suggesting the existence of single domain states in these smaller diameter nanowires. Magnetic force microscopy measurements confirmed the magnetic domain structures suggested by magnetic hysteresis measurements. Similar investigations of Fe nanowires with diameters of 250 nm and 60 nm found that they all have multidomain magnetic structures. This is expected based on their material properties and polycrystalline structures. Furthermore, magnetic structures of Y-branches and multi-wire clusters were also studied using magnetic force microscopy. The as-prepared Ni-P nanotubes had an amorphous structure. Following a heat treatment, however, a structural phase transformation from the amorphous phase to a crystalline phase was observed using X-ray diffraction measurements. The tetragonal crystalline phase of Ni3P and the face-centered-cubic phase of Ni were confirmed via simulations by the GSAS software. The high Ni3P content accounts for the semiconducting behavior and a low magnetic anisotropy observed in the Ni-P nanotubes.
402

Modified fermi-eyges electron scattering in tissue equivalent media

Blais, Noël January 1990 (has links)
A theoretical and experimental study of the spatial spread of electron dose distributions for pencil and broad electron beams in various media is presented. We discuss in detail two methods for measurement of electron beam kinetic energies, describe our technique for the measurement of the optical density versus dose relationship for radiographic films, and present the experimental set-up for the measurement of the spatial spread of electron dose distributions for pencil and broad electron beams. / The Fermi-Eyges theory for the small angle multiple Coulomb scattering describes the spatial electron distribution in scattering media. The spatial spread of a pencil electron beam in a phantom as predicted by the Fermi-Eyges theory is an increasing function of depth in phantom irrespective of the depth. Our experiments, on the other hand, show that the spatial spread indeed increases with depth until depths close to 2/3 of the practical electron range, but at larger depths the spatial spread saturates, then decreases and vanishes at depths greater than the range of electrons in the material. To describe the observed saturation and the decrease of the spatial spread at depths beyond 2/3 of the practical electron range we introduce an electron absorption term containing four empirical parameters into the original Fermi differential equation and show that its solution describes the experimental results obtained for pencil electron beams of various energies in polystyrene, cork and aluminum phantoms. We also compare with experimental results the spatial electron dose distributions predicted by the Fermi-Eyges theory and the modified Fermi-Eyges theory for collimated broad electron beams.
403

New ternary rare-earth antimonides and germanides: bonding, structures, and physical properties

Bie, Haiying 11 1900 (has links)
This thesis focuses on the synthesis, structures, and physical properties of ternary rare-earth antimonides and germanides. These ternary compounds exhibit diverse polyanionic substructures with classical and non-classical Sb–Sb bonding, Ge–Ge bonding, or both. The Zintl-Klemm concept and band structure calculations were applied to understand their structures and bonding. Electrical resistivities and magnetic properties were measured for these compounds. The compounds RE2Ti7Sb12 (RE = La–Nd) and RE2Ti11–xSb14+x (RE = Sm, Gd, Tb, Yb), which were synthesized by arc-melting, adopt different structures depending on the size of the RE atoms. Both consist of a complex arrangement of TiSbn polyhedra, linked to form a 3D framework with large cavities in which the RE atoms reside. Hypervalent Sb–Sb bonds are manifested in disordered Sb fragments in RE2Ti7Sb12, and 1D linear chains, zig-zag chains, and pairs in RE2Ti11–xSb14+x. A series of compounds, RECrGe3 (RE = La–Nd, Sm), was synthesized by the Sn-flux method. They adopt a hexagonal perovskite structure type, in which chains of face-sharing Cr-centred octahedra are linked by triangular Ge3 clusters. These unusual single-bonded Ge3 substructures can be rationalized simply by the Zintl-Klemm concept. Electrical resistivity measurements show metallic behaviour with prominent transitions coincident with ferromagnetic transitions (Tc ranging from 62 to 155 K) found in magnetic measurements. Band structure calculations show the presence of a narrow, partially filled band with high DOS at Ef, in agreement with the observation that LaCrSb3 is an itinerant ferromagnet. With a different number of d-electrons in the M site, the isostructural REVGe3 compounds exhibit antiferromagnetic behaviour. The doped quaternary compounds LaCr1-xVxGe3 and LaCr1-xMnxGe3 exhibit depressed Curie temperatures. The structures of RECrxGe2 compounds (RE = Sm, Gd–Er) are built up by inserting transition-metal atoms into the square pyramidal sites of a hypothetical “REGe2” host structure (ZrSi2-type). The presence of extensive anionic Ge substructures in the form of 1D zigzag chains and 2D square sheets can be explained by the Zintl-Klemm concept. Magnetic measurements indicated antiferromagnetic ordering with low TN ranging from 3 to 17 K. Compounds involving a p-block element as the second component were prepared. In RE12Ge7–xSb21 (RE = La–Pr), a complex 3D polyanionic framework with Ge pairs, five-atom-wide Sb ribbons, and 2D Ge/Sb layers is present. The bonding exemplifies the competition of valence electron transfer from the RE atoms to metalloids with similar electronegativities. Full electron transfer from the RE atoms to the anionic substructure cannot be assumed. Magnetic measurements on Ce12Ge6.5Sb21 indicate antiferromagnetic coupling. The metal-rich compounds RE5TtxSb3–x (Tt = Si, Ge) adopt the orthorhombic beta-Yb5Sb3-type structure with a range of solid solubility between 0.9 and 1.6. They are not electron-precise and do not obey the Zintl-Klemm concept.
404

Magnetism and structure in metallic multilayers /

Holmström, Erik, January 2003 (has links)
Diss. (sammanfattning) Uppsala : Univ., 2003. / Härtill 17 uppsatser.
405

Analysis of the demagnetisation process and possible alternative magnetic treatments for naval vessels /

Baynes, Timothy Malcolm. January 2002 (has links)
Thesis (Ph. D.)--University of New South Wales, 2002. / "a thesis submitted for the degree of Doctor of Philosophy in the School of Physics, Faculty of Science at the University of New South Wales" Also available online.
406

An investigation into the use of electrodynamic wheels for high-speed ground transportation

Bird, Jonathan. Unknown Date (has links)
Thesis (Ph.D.)--The University of Wisconsin - Madison, 2007. / (UMI)AAI3261460. Source: Dissertation Abstracts International, Volume: 68-04, Section: B, page: 2532. Adviser: Thomas A. Lipo.
407

Spin dynamics of quantum spin-ladders and chains /

Notbohm, Susanne. January 2007 (has links)
Thesis (Ph.D.) - University of St Andrews, October 2007.
408

Critical spin dynamics in anisotropic magnets

Raghavan, Raghu Pati, January 1976 (has links)
Thesis--Wisconsin. / Vita. Includes bibliographical references (leaves 117-121, 16-17).
409

Temperature-induced phenomena in systems of magnetic nanoparticles

Bhuiya, Abdul Wazed. January 2009 (has links)
Thesis (Ph. D.)--University of Texas at El Paso, 2009. / Title from title screen. Vita. CD-ROM. Includes bibliographical references. Also available online.
410

Multilevel multipole-free fast algorithm for electromagnetic scattering problems in layered media /

Saville, Michael Andrew. January 2006 (has links)
Thesis (Ph.D.)--University of Illinois at Urbana-Champaign, 2006. / Source: Dissertation Abstracts International, Volume: 67-11, Section: B, page: 6633. Adviser: Weng Cho Chew. Includes bibliographical references (leaves 143-146) Available on microfilm from Pro Quest Information and Learning.

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