1,721,032 research outputs found
Physics of iron and earth's inner core.
No full textThe Earth's core is primarily composed of iron and I studied the physics of this element. This dissertation addresses two important questions concerning the physical properties of hexagonally close packed (hcp) iron, the high pressure polymorph. In the first part (chapter II and III) the focus is on the thermoelastic properties of hcp iron at the thermodynamic conditions of the deep interior of the Earth, advancing our understanding of inner core processes and structure. Combining accurate band structure total energy methods for electronic contributions to the Helmholtz free energy with a mean field approach to the lattice vibrations, I have predicted the structure and elastic constants of dense hcp iron at high temperature from first principles. The axial ratio c/ a in the hcp cell increases substantially with temperature, reaching a value of nearly 1.7 at 5700 K, where aggregate elastic properties match those of the inner core. As a consequence of increasing c/ a the single crystal longitudinal anisotropy at high temperature has the opposite sense from that at low temperature. Combining the single crystal elasticity with a simple model of inner core texture, in which basal planes are preferentially aligned with Earth's rotation axis, one can account for seismological observations of inner core anisotropy. The second part (chapters IV and V) investigates the effect of a theoretically predicted anti-ferromagnetic ground state at pressure on spectroscopic and elastic properties, the results adding to a growing body of theoretical evidence for magnetism in hcp iron at pressure. The orthorhombic anti-ferromagnetic ordering leads to two Raman active transverse optical modes, as found experimentally, while only one such mode is predicted in the hcp structure. Moreover, we find good agreement with experimental observations of the frequencies of the two modes. The influence of anti-ferromagnetic ordering on the elasticity of hcp iron reduces the discrepancy between theory and experiment in equation of state and aggregate elastic properties.PhDEarth SciencesGeophysicsUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/128722/2/3029439.pd
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Chemical Reactions at the Earth's Core-Mantle Boundary
Get PDFEarth’s evolution and structure depends heavily on the composition of its interior and the processes occurring deep within. The main objective of this thesis is to better understand the nature of the core-mantle boundary, which not only provides insight for the Earth’s interior, but can also be related to other planetary body interiors as well. At the core-mantle boundary, two major planet forming materials (rock and metal) are juxtaposed at high temperatures where the interaction of the materials may occur. Characterizing this interaction requires investigating any chemical reactions that may occur and the possible transport of heat and mass between these two regions. The behavior of this boundary layer can be directly related to understanding deep Earth dynamics, including the origin of the early dynamo, observed seismic structures in the deep mantle, and the redox chemistry of Earth. The work in this thesis outline the different aspects of my Ph.D. projects that work to investigate the nature of the core-mantle boundary. This not only includes the possible interaction of existing mantle and core, but also the behavior of materials introduced to this region by tectonic activity originating from the Earth’s surface such as H2O, and the interaction of this other important planetary component (ice) with rock and metal. I apply first principles molecular dynamics to investigate the chemical reactions of rock and metal materials in the Earth’s mantle. I also investigate the material properties of two FeOOH polymorphs, ε-FeOOH and pyrite-structured FeOOH, using first principles static simulations that perform ground state calculations to determine phase stability, elasticity, and phonon vibrational frequencies
Mineral physics of the mantle transition zone: Solid solutions and elasticity.
No full textOne goal of high pressure mineralogy is to determine the composition of the earth's interior. This effort must include an understanding of elasticity at high pressures, and the effects of disorder and of solid solutions on physical properties of minerals. Chapters II and III focus on wadsleyite, which is thought to be the most abundant mineral in the upper part of the earth's transition zone (410--520 km depth). The calculated elastic anisotropy of this phase is small and almost independent of pressure. The results also show that bond length and bond-angle systematics in silicates obtained from chemical variations at zero pressure do not capture the effect of pressure correctly. Wadsleyite shows a non-random iron distribution on its three symmetrically distinct octahedral sites. The results (Chapter III) provide an explanation for this observation in terms of crystal field splitting of the highly acentric M2-site. For ringwoodite, which is thought to be most abundant mineral in the lower part of the transition zone (520--660 km depth) the energetics of magnesium/silicate disorder at high pressures is investigated (Chapter IV). The results show the importance of structural relaxation for the modeling of Mg/Si disorder. The enthalpy differences are a factor 80 smaller as compared to previous studies but still too high to stabilize completely inverse ringwoodite at transition zone conditions. Ferromagnesian silicate perovskite is thought to be the most abundant mineral in the earth's lower mantle (660--2900 km depth) and should dominate the physical properties in this region. Seismic tomography commonly observes an anti-correlation of lateral variations in shear-wave and bulk-sound speed in the earth's deep lower mantle. It was suggested that lateral variations of iron content in perovskite could explain this observation. The results presented in Chapter V show that this is not the case and other explanations must be sought.PhDEarth SciencesGeophysicsUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/132266/2/3057985.pd
Fluid and rock interactions in silicate and aluminosilicate systems at elevated pressure and temperature.
No full textUnderstanding fluid chemistry in the subduction zone environment is key to unraveling the details of element transport from the slab to the surface. Solubilities of cations, such as silicon, in water strongly affect both the physical and chemical properties of supercritical metamorphic fluids. Modeling the thermodynamics of fluid-rock interactions requires therefore a profound understanding of cation dissolution and aqueous speciation. In situ Raman experiments of the silica-water, alumina-water, and alumina water systems were performed in an externally heated Bassett-type diamond-anvil cell at the Department of Geological Sciences, University of Michigan. Natural quartz samples and synthetic ruby samples were used in the experiments. Samples were loaded in the sample chamber with a water pressure medium. All experiments used rhenium gaskets of uniform thickness with a 500 mum drill hole for the sample chamber. Temperature was measured using K-type thermocouples encompassing both the upper and lower diamond anvils. Pressures are obtained on the basis of the Raman shift of the 464 cm-1 quartz mode where possible or the Raman shift of the tips of the diamond anvils according to a method developed in this work. This work characterizes the state of stress in the diamond anvil cell, which is used as the basis for the pressure calibration using only the diamond anvils. Raman measurements of silicate fluid confirm the presence of H4 SiO4 and H6Si2O7 in solution and expand the pressure range for in-situ structural observations in the silica-water system. Additionally, we identify the presence of another silica species present at mantle conditions, which occurs at long time scales in the diamond cell. This study provides the first in situ data in the alumina-water and alumina-silica-water systems at pressures and temperatures relevant to the slab environment. Al(OH) 3 appears to be the dominant form of alumina present under these conditions and in the alumina-silica-water system exists simultaneously with a silica monomer species. Raman peaks also suggest the presence of an aqueous alumina-silica dimer specie.PhDEarth SciencesGeophysicsMineralogyUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/125058/2/3186610.pd
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Get PDF“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Get PDFWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Dispelling the Myths Behind First-author Citation Counts
Get PDFWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
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