1,721,107 research outputs found
EXAFS studies of dopant sites in metal oxides
No full textThe application of EXAFS to the study of location cation dopants in metal oxide lattices is described by illustrative examples. The examples are taken from studies of optoelectronic (LiNbO3), sensors (SnO2) and solid electrolyte (Bi2O3) materials. A brief description is given of the experimental techniques and the methods of data analysis
A study of the sulfur problem in Mary Rose timbers using X ray technique
Full text linkThis thesis reports a study of the sulfur problem which is the production of sulfuric acid via the oxidation of iron sulfide in the timbers of the Mary Rose a flagship of Henry VIII 's navy which sunk in 1535 and was raised in 1982. The work has involved a range of chemical and physical techniques with particular use of synchrotron sources to measure the iron and sulfur speciation in the timbers with X-ray absorption spectroscopy (XAS) measurements. XAS measurements are almost unique in providing the speciation of atoms in a sample via the XANES. They are particularly useful for sulfur, which has a wide range of oxidation states. However, the current work has shown the need to use the bulk and microfocus XAS measurements in parallel, particularly for archaeological samples. It is clear that the iron and sulfur contents and speciation can vary widely from sample to sample of the Mary Rose timbers. In the study on the effectiveness of chelating agents in the removal of iron species from the timbers care was taken, wherever possible, to ensure that the same samples and sample positions were used for the before and after treatment measurements. The nature of the iron and su~ur species is of extreme importance because it is assumed that it is Fe" that gives rise to the production of su~uric acid. The current study has shown that in the timbers that had not been PEG treated contained iron in the surface regions that was predominantly Fe", similar to the findings of other workers. Most of the work in this thesis used samples close to the surface of the timbers. It was only in samples taken deep into the timbers experiments that there were significant concentrations of Fe". A range of sulfur species were found in the samples. The predominant species were reduced sulfur species, elemental sulfur and sulfate. Very little pyrite was found -in the timbers studied, but it should be noted that these timbers had not been PEG treated. Some pyrite was found in the cell walls. The present studies were predominantly on the surface regions of the timbers and the conclusion is that the bulk of the pyrite which may have been present had oxidised in the moist, oxygen containing environment in which they had been stored after recovery from the sea bed. A key finding of the present study is the co-location of iron and sulfate in the timbers. This had been suggested but had not been experimentally verified. The production of sulfuric acid in the timbers is thought to involve the oxidation of iron sulfides in the presence of water to produce is iron sulfate and sulfuric acid. The fact that the present experiments show iron and sulfate in the same positions i'] the XANES maps strongly supports the proposed oxidation mechanism of iron sulfides. The bulk and microfocus XAS experiments show that a large fraction of the iron in the current samples was in the form of an oxide. This is most likely to be goethite (FeO(OH)). The XANES analysis and the fitting of the EXAFS are consistent with this identification. All the four chelating agents used in this work (EDTA, DTPA, ammonium citrate and calcium phytate) were effective in removing iron from the timbers. However, the more efficient are DTPA and calcium phytate in terms of amount removed at fixed molarity. For samples that had been treated with PEG the current work showed that the chelating agents were less effective. This is presumably due to the PEG blocking the penetration of the solutions of the chelating agents into the wood
Studies of methods to restrict the grain growth of nanocrystalline metal oxides
Full text linkThere is considerable interest in nanocrystalline materials. This thesis is concerned with nanocrystalline oxides and the development of methods to prevent their grain growth on heating. This growth, which is evident at temperatures as low as 400°C, presents a serious problem in the study and applications of nanocrystalline oxides. The systems that were studied were nanocrystalline magnesium oxide, zirconium oxide, cerium oxide and tin oxide. The methods of preventing grain growth included the encapsulation of the oxide in the pores of porous silica, mixing with nanocrystals of alumina and treating the surface with a silanising agent, hexamethyldisilazane. All the methods employed showed some effect on reducing the grain growth. Encapsulation in the pores of silica was effective, however it proved difficult to get large amounts of the oxides into the pores. A more efficient method of preparing large samples was the incorporation of alumina, which was achieved by a sol-gel process. An alkoxide of the target oxide and an aluminium alkoxide were mixed and then hydrolsed and calcined. This proved very effective for magnesium oxide, zirconium oxide and tin oxide. For example, heating zirconium oxide at 1000°C for 60 minutes causes the nanocrystals to grow to about 50nm. Treatment with alumina restricts the growth to 12nm. Similar effects were found for the other oxides, although magnesium oxide showed a reaction with alumina at the highest temperatures. Silanising the surface was only studied for tin oxide and it restricted growth at 1000°C to 27nm, compared to 88nm for an untreated sample. A full description is given of the preparative methods and structural studies of the systems using X-ray diffraction and X-ray absorption spectroscopy
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
Coherent diffuse neutron scattering study of vacancy correlations in yttria-stabilised zirconia
No full textVariations 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
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
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe 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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