1,720,964 research outputs found
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe 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
Full text link“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
Characterization of crystalline and amorphous phases in poly(N-methylethylenimine) polymer electrolytes and model systems.
Full text linkLPEI is analogous to PEO, except LPEI has a heteroatom that is synthetically versatile because different side groups can be readily attached. LPEI:salt systems are not well understood as polymer electrolytes. Therefore, N, N '-dimethylethylenediamine (DMEDA) complexed with NaCF3 SO3 (NaTf) was investigated in this dissertation as a model compound for LPEI, in order to gain further insight into hydrogen bonding in amine systems. While studying DMEDA:NaTf, single crystals were discovered and characterized with infrared and Raman spectroscopy, X-ray diffraction, and differential scanning calorimetry (DSC). In DMEDA:NaTf solutions, there is evidence of local structures similar to that in crystalline DMEDA:NaTf based on infrared and Raman spectroscopy data.Polymer electrolytes have been investigated as ionic conductors in rechargeable lithium batteries. Poly(ethylene oxide):salt systems are the most widely investigated polymer electrolytes. However, the conductivity values for these system are not sufficiently high to be useful for practical lithium batteries. Therefore, other polymer electrolytes are being explored such as linear poly(ethylenimine), LPEI, and linear poly(N-methylethylenimine), LPMEI.LPMEI is a derivative of LPEI. LPMEI is amorphous at room temperature unlike LPEI, which melts at 58° C. Because there is a limited amount of information reported about LPMEI as polymer electrolytes, this dissertation characterizes LPMEI complexed with LiTf and NaTf utilizing IR, differential scanning calorimetry (DSC), powder X-ray diffraction, and complex impedance. To gain further insight into LPMEI systems, model systems were investigated such as N, N, N', N'-tetramethylethylenediamine (TMEDA) and N, N, N', N', N' -pentamethyldiethylenetriamine (PMDETA) complexed with LiTf and NaTf. When studying these model systems, crystalline compounds were discovered and characterized utilizing X-ray diffraction, DSC, IR, and Raman spectroscopy. The local structures in crystalline TMEDA:LiTf, TMEDA:NaTf, and PMDETA:NaTf can be correlated to these samples in solution, which provides greater insight into the local environment of the LPMEI:salt systems
MICROCRYSTALLINE REFLECTIVITY IN UNIAXIAL CRYSTALS
No full textAuthor Institution: Department of Chemistry, University of OklahomaThe infrared reflectivity of a uniaxial microcrystalline powder is described as a composite reflectivity from various faces which have the proper alignment with respect to the incident beam. The optical constants of the microcrystal are assumed to be the bulk crystal values and the various faces presented to the incident beam are obtained from a simple rotational averaging. The Fresnel reflection coefficients for an absorbing uniaxial crystal are given for an arbitrary orientation of the optic axis with respect to the plane of incidence. Applications to experimental data are discussed
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe 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
Vibrational spectroscopy of phosphate-based electrodes for lithium rechargeable batteries.
Full text linkThe favorable energy densities and cycling performances attainable from lithium rechargeable batteries has prompted aggressive research in this field for over 30 years. In particular, phosphate-based electrodes receive much attention due to their competitive redox potentials, capacities, and enhanced safety compared to currently commercialized systems. In developing these materials, it is critical to have a good understanding of the fundamental reactions that occur during cycling. Vibrational spectroscopy is particularly useful in this regard because it is sensitive to the local structure of the electrodes. However, few researchers use vibrational spectroscopy to study lithium intercalation in phosphate-based electrodes. The intramolecular vibrations of the phosphate anions yield intense bands in the Raman and infrared spectrum that are sensitive to the immediate local environment of the phosphate anions; thus they are natural candidates for the technique. Measuring the infrared or Raman spectrum of these materials at various states of charge can provide a wealth of information about local structural changes and charge compensation reactions occurring within an electrode from the perspective of the phosphate anions.The first objective of this dissertation is to examine lithium intercalation with vibrational spectroscopy for three families of phosphate-based electrodes: phospho-olivine LiMPO4 (M = Fe, Mn) cathodes, LISICON Li3 M2(PO4)3 (M = V, Ti) cathodes, and LiSn2(PO4)3 and Sn3(PO4) 2 anodes. Symmetry-based analyses are used to interpret the vibrational modes of each compound. Vibrational spectra recorded at various states of charge demonstrate the sensitivity of the phosphate anions to the presence of lithium ions and provide insight into the lithium intercalation reactions that occur within these materials. The second objective of this dissertation is to describe new in situ electrochemical cells for Raman and infrared spectroscopy. For example, a relatively cheap in situ Raman cell based on modified coin cells is tested with V2O 5 cathodes. The field of in situ infrared spectroscopy is extended to include transmission experiments. The performance of a new in situ transmission FT-IR spectroelectrochemical cell is tested with FePO4 cathodes, and the results agree very well with ex situ experiments. Both techniques provide new ways to study lithium intercalation for a range of advanced electrode materials
Local structures and conductivity in polyethylenimine and polyphosphazene polymer electrolytes.
No full textLinear polyethylenimine (LPEI) is structurally analogous to poly(ethylene oxide), with an N-H group in place of the ether oxygen. LPEI systems with dissolved lithium and sodium triflate were prepared and characterized via Raman and FT-IR vibrational spectroscopy, differential scanning calorimetry, and complex impedance. The understanding of local interactions in the LPEI-salt systems is enhanced by consideration of a small-molecule model of LPEI, N-N '-dimethylethlenediamine (DMEDA). Vibrational spectroscopy of DMEDA-lithium salt systems was used to elucidate ion-ion interactions and the interaction of the solvent with the salt. Mode assignments were made in the DMEDA system. It was shown that the vibrational modes from 700 to 900 cm-1, which are a mixture of CH2 rock and NH bend, are sensitive to interaction with the lithium cation. It was found from the frequency shifts in the NH stretching region that LPEI is more extensively hydrogen bonded than DMEDA and that in both systems hydrogen bonding is decreased when LiTf is added. It was also found that the vibrational modes of the triflate anion are influenced by solvent interactions in the LPEI and DMEDA systems, probably via a hydrogen bonding interaction.Poly(bis-methoxyethoxyethoxyphosphazene), MEEP, has the same coordinative moiety (CH2CH2O) as is found in the poly(ethyleneoxide) system, but attached to a flexible phosphazene backbone. A spectroscopic comparison of MEEP and three other phosphazenes with ethoxy sidechains of different lengths shows that the dissolved lithium cation coordinates primarily to the ether oxygens in these systems. A comparison of ionic speciation to the concentration-dependant conductivity in the MEEP-lithium triflate system has led to the proposal of a mechanism of ionic conductivity involving associated ionic species via dissociation and rapid reassociation.Polymer electrolytes are ionic conductors which consist of a salt dissolved in a polymer host. The potential importance of polymer electrolytes in applications such as rechargeable lithium batteries has stimulated intense interest in these systems. Extensive fundamental investigations into the nature of ionic conductivity in these systems have previously been done on poly(ethylene oxide) with dissolved lithium triflate, LiCF3SO3. These investigations are now complemented by considering related polymer-salt systems. This dissertation investigates two such systems
Electrode materials for lithium rechargeable batteries: Synthesis, spectroscopic studies and electrochemical performance.
Full text linkThree distinct \rm Li\sb{x}V\sb2O\sb5 phases, and -\rm Li\sb{x}V\sb2O\sb5, were obtained through a chemical intercalation reaction and solid state reactions. Infrared and Raman spectra were recorded for the three phases. The spectral changes were interpreted in terms of the local structural changes of the vanadium-oxygen polyhedra. Although the and phases have very similar powder x-ray diffraction patterns, IR and Raman studies showed these two phases adopt distinctive local structural environments. These results demonstrate that IR and Raman spectroscopy are important techniques for the structural analysis of intercalation materials.For the first time novel mesostructural materials were synthesized as electrode materials for the lithium rechargeable battery. The well-ordered mesostructural materials provide an ideal host for lithium transport processes. The preliminary results on the manganese oxide-based cathode and tin oxide-based anode show that the templating synthesis technique may provide important electrode materials for battery applications.In situ Raman spectra of \rm Li\sb{x}V\sb2O\sb5 were successfully recorded on a operating lithium rechargeable battery. Distinctive spectral changes were observed at different lithium intercalation levels and interpreted in terms of the slight rearrangements of the V-O structural units. The results show that in situ Raman spectroscopy may become an important nondestructive technique in investigating the irreversible structural changes in electrode materials and evaluating battery performance.Single crystals of \rm Li\sb{1.1}V\sb3O\sb8 and \rm\sp6Li\sb{1.1}V\sb3O\sb8 were prepared using solid state synthesis techniques. IR spectra and polarized Raman spectra were recorded on the \rm Li\sb{1.1}V\sb3O\sb8 and \rm\sp6Li\sb{1.1}V\sb3O\sb8 crystals and a lithiated phase, \rm Li\sb4V\sb3O\sb8. Factor group analysis method was used to interpret the spectral changes. These spectroscopic results provide insight into the structural modifications originating from lithium intercalation/deintercalation processes.The lithium rechargeable battery is the newest member of the rechargeable battery family and is best known for its high energy density, long battery life, low self-discharge rate and light weight. This battery may become one of the most important energy sources in consumer market, industrial and military applications. Intercalation compounds play a critical role in determining the overall performance of a lithium rechargeable battery. The common intercalation materials for battery applications are layered structure \rm Li\sb{x}CoO\sb2, spinel \rm Li\sb{x}Mn\sb2O\sb4 and lithium vanadium oxides, \rm Li\sb{x}V\sb2O\sb5 and $\rm Li\sb{x}V\sb3O\sb8.
INTRAMOLECULAR MODE MIXING IN AND CRYSTALS
No full textAuthor Institution:The molecular anions and exhibit symmetric and a symmetric intramolecular stretching vibrations, and , which transform according to the and E irreducible representations of the point group. However in and , crystals (space group ) the multiply occupied unit cells result in factor group components of F symmetry which originate in the symmetric and asymmetric at stretching vibrations. These components are sufficiently close in frequency so that intramolecular mode mixing is significant. This effect is investigated in terms of convenient model and the infrared and Raman spectra of the vibrational multiplet structure in anion stretching region are analyzed
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