1,721,301 research outputs found
Synthesis, and/or Structural, Conductivity Investigations of Single and Polycrystalline Fluorides, Ionic Liquids and Conjugated Diynes Using Solid-state Nuclear Magnetic Resonance Spectroscopy and X-ray Diffraction
No full text244 pg.One overarching focus of solid-state chemistry has evolved due to the desire to understand the structures and mechanisms underlying conductivity, whether electronic or ionic, and to seek ways toward enhancement and control of those mechanisms and materials. Modern strategies are also shaped by environmental concerns and these concerns have, in turn, influenced the search for synthetic routes which employ more benign methods. The first sections of this dissertation describe adapted and modified reverse-micellar methods, as applied toward the room-temperature synthesis of several different types of binary and lanthanide-doped, monodisperse, nanoparticle (20-40nm), polycrystalline fluorides. Their structures were investigated using Magic-angle Spinning Nuclear Magnetic Resonance (MAS NMR) and X-ray Crystallography. The difficulty of applying these aqueous methods toward nanoparticle lead fluoride (PbF2) was surmounted when a novel crystal, cetyltrimethylammonium hexafluorosilicate monohydrate, was synthesized, its crystal structure solved, and employed as a fluoride delivery mechanism to successfully synthesize both a- and b-phase PbF2. MAS NMR spectroscopy is well-suited as an investigative tool for both crystalline and non-crystalline materials. Two sections of the dissertation deal with two types of compounds in which MAS NMR multinuclear pulse techniques (19F, 7Li, 1H, and 13C) play a critical role in the deduction of structure (conjugated diynes) and conductive behavior as a function of temperature (ionic liquids). The final sections of this dissertation again rely heavily on X-ray crystallography and MAS NMR, as well as Impedance Spectroscopy and EXAFS/XANES via collaborative efforts, to examine fluoride conductivity and temperature-dependent behavior of fluoride materials. The super-Lewis acid, antimony pentafluoride, was used to explore whether morphology affects mobility across grain boundaries when vacancies are artificially induced in both nanoparticles as well as their larger polycrystallite analogs. In a collaborative effort, barium fluoride and calcium fluoride single-crystal heterostructures were grown by the Joachim Maier Group and were used to elucidate the precise mechanisms of observed conductivity enhancement in these heterostructures.Advisor(s): Grey, Clare P. Committee Member(s): Lauher, Joseph ; Greenbaum, Steven G; White, Michael.Stony Brook University Libraries. SBU Graduate School in Department of Chemistry. Charles Taber (Dean of Graduate School)
A Solid State NMR and EXAFS Study of Manganese Oxide Minerals
No full text175 pg.Though only a minor component of soils occurring as thin coatings on rocks and sediments, manganese oxide minerals are excellent sorbents of various cations and have a significant impact on the geochemical cycling of environmentally relevant cations such as Pb, Cd, Ni, Cu and Zn. The layered manganese oxide, birnessite, has been found to have the highest sorption capacity of these Mn minerals, attributed to the negative layer charge that arises from structural "defects" such as layer Mn3+ ions and Mn4+ vacancies, as well as the high interlayer surface area available for the sorption of charge compensating cations. Great interest in understanding the nature of cation adsorption in these materials has resulted in numerous cation sorption studies, utilizing techniques to extract information regarding locations and ordering of sorbed cations and quantifying layer Mn vacancies. However, commonly utilized techniques such as EXAFS and X-ray diffraction have high errors associated with the refinement of cation coordination and quantification of layer vacancies, and require correlation of results from other techniques to support these refinements. Application of solid state NMR to these materials has not been extensively explored due to difficulties associated with the paramagnetic manganese ions. This dissertation aims at illustrating the utility of this technique in these paramagnetic materials. NMR is sensitive to the local environment, including the types and oxidation states of coordinated metals, coordination geometry and bonding distances surrounding the observed nucleus. 23Na NMR is used to determine sodium locations with respect to the ordering of Mn3+ and Mn4+ in the triclinic form of birnessite and other layered sodium manganese oxides, indicating layer charge distribution drives cation ordering. 2H NMR is used to understand the interactions of crystalline interlayer water with that of layer vacancies in hexagonal birnessite, and to quantify layer vacancies that are charge compensated by deuterons in these materials. Development of a special tailored structural model of Mn K-edge EXAFS data for these hexagonal birnessites support the trends observed in the 2H NMR. The use of solid state NMR in these paramagnetic systems enriches our understanding of the complex interactions between the Mn layers and interlayer species in these important manganese oxide minerals.Advisor(s): Grey, Clare P.. Committee Member(s): Koch, Stephen ; Mayr, Andreas ; Smith, Luis.Stony Brook University Libraries. SBU Graduate School in Department of Chemistry. Charles Taber (Dean of Graduate School)
Current challenges and routes forward for nonaqueous lithium–air batteries
No full textNonaqueous lithium-air batteries have garnered considerable research interest over the past decade due to their extremely high theoretical energy densities and potentially low cost. Significant advances have been achieved both in the mechanistic understanding of the cell reactions and in the development of effective strategies to help realize a practical energy storage device. By drawing attention to reports published mainly within the past 8 years, this review provides an updated mechanistic picture of the lithium peroxide based cell reactions and highlights key remaining challenges, including those due to the parasitic processes occurring at the reaction product-electrolyte, product-cathode, electrolyte-cathode, and electrolyte-anode interfaces. We introduce the fundamental principles and critically evaluate the effectiveness of the different strategies that have been proposed to mitigate the various issues of this chemistry, which include the use of solid catalysts, redox mediators, solvating additives for oxygen reaction intermediates, gas separation membranes, etc. Recently established cell chemistries based on the superoxide, hydroxide, and oxide phases are also summarized and discussed.</p
Two electrolyte decomposition pathways at nickel-rich cathode surfaces in lithium-ion batteries.
Get PDFPreventing the decomposition reactions of electrolyte solutions is essential for extending the lifetime of lithium-ion batteries. However, the exact mechanism(s) for electrolyte decomposition at the positive electrode, and particularly the soluble decomposition products that form and initiate further reactions at the negative electrode, are still largely unknown. In this work, a combination of operando gas measurements and solution NMR was used to study decomposition reactions of the electrolyte solution at NMC (LiNi x Mn y Co1-x-y O2) and LCO (LiCoO2) electrodes. A partially delithiated LFP (Li x FePO4) counter electrode was used to selectively identify the products formed through processes at the positive electrodes. Based on the detected soluble and gaseous products, two distinct routes with different onset potentials are proposed for the decomposition of the electrolyte solution at NMC electrodes. At low potentials (80% SOC), 1O2 released from the transition metal oxide chemically oxidises the electrolyte solvent (EC) to form CO2, CO and H2O. The formation of water via this mechanism was confirmed by reacting 17O-labelled 1O2 with EC and characterising the reaction products via 1H and 17O NMR spectroscopy. The water that is produced initiates secondary reactions, leading to the formation of the various products identified by NMR spectroscopy. Noticeably fewer decomposition products were detected in NMC/graphite cells compared to NMC/Li x FePO4 cells, which is ascribed to the consumption of water (from the reaction of 1O2 and EC) at the graphite electrode, preventing secondary decomposition reactions. The insights on electrolyte decomposition mechanisms at the positive electrode, and the consumption of decomposition products at the negative electrode contribute to understanding the origin of capacity loss in NMC/graphite cells, and are hoped to support the development of strategies to mitigate the degradation of NMC-based cells
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
- …
