1,721,054 research outputs found
Characterization of natural porous media by NMR and MRI techniques : high and low magnetic field studies for estimation of hydraulic properties
No full textSoil is the natural material that covers most of the dry surface of the earth. It is the product of mechanical, chemical and biological interactions of different types of elements. Soils have a very complex composition and high variability in their occurrence and properties. The non-homogeneous mixture and interaction of their components and the changes induced upon usage ensures the complexity of their structure and a comfortable variable spatial distribution over the surface of the planet. The soils components can be found in nature in all the three aggregation states of matter: the solid state, the liquid state and the gas state. In spite of their complex structure and composition, soils can be easily handled and studied by a variety of methods. Prediction of water movement in soils is a controlling factor in various processes of interest in water resources management such as: the runoff generation, the water and nutrients supply to vegetation, the groundwater recharge and contamination. From the physics point of view water distribution and transport in unsaturated soil represents a rather complex problem of porous media hydraulics. Among various techniques developed for investigating hydraulic phenomena in soils, Nuclear Magnetic Resonance (NMR) can be used successfully for the characterization of natural porous media. The aim of this thesis is to apply different NMR techniques for: i) understanding the relaxometric properties of unsaturated natural porous media and ii) for a reliable quantification of water content and its spatial and temporal change in model porous media and soil cores. For that purpose, porous media with increasing complexity and heterogeneity were used (coarse and fine sand and different mixture of sand/clay) to determine the relaxation parameters in order to adapt optimal sequence and parameters for water imaging. Conventional imaging is mostly performed with superconducting high field scanners but low field scanners promise longer relaxation times and therefore smaller loss of signal from water in small and partially filled pores. By this reason high and low field NMR experiments were conducted on these porous media to characterize the dependence on the magnetic field strength. Correlations of the NMR experiments with classical soil physics method like mercury intrusion porosimetry; water retention curves (pF) and multi-step-outflow (MSO) were performed for the characterization of the hydraulic properties of the materials. Due to the extensive research the experiments have been structured in three major parts as follows. In the first part a comparison study between relaxation experiments in high and low magnetic field was performed in order to observe the influence of the magnetic field on the relaxation properties. Due to these results, in the second part of the study only low field relaxation experiments were used in the attempt of correlations with classical soil physics methods (mercury intrusion porosimetry and water retention curves) for characterizing the hydraulic behavior of the samples. Further, the aim was to combine also MRI experiments (2D and 3D NMR) with classical soil physics methods (multi-step-outflow, MSO) for the same purpose of investigating the hydraulic properties. Because low field MRI systems are still under developing for the moment, the MRI experiments were performed in high magnetic field and combined with MSO experiments in order to asses the changes in water content over the depth of the samples during pressure application, changes that are governed by the hydraulic properties of the material. The thesis is structured into 7 chapters. First chapter is an introduction to actual state of NMR research in soil science. The second and the third chapters are theoretical consideration over the NMR and soil physics methods that have been used for investigations. Nevertheless, only the basic theoretical aspects are covered here since due to extensive research we find it more appropriate to introduce each method used, separately, further on, into the content of the thesis. The materials and methods used for investigation differ from one part of the research to another; therefore they will be discussed in detail for each chapter separately. Chapter 4 is the comparison study between high and low field NMR; in chapter 5 is described the combination of low field NMR relaxometry with soil physics methods; chapter 6 presents the MRI studies on a coaxial sample and the parameterization of the water retention functions using a combination of MRI and MSO experiments. In chapter 7 a summary of the entire research is performed and the main conclusions are presented together with an outlook for further researches based on the obtained results
Molecular dynamics of polymers by means of NMR field cycling relaxometry
No full textThe molecular dynamics of polymer melts and networks with molecular weight well above the critical value were studied based on a power law dependence of the NMR spin-lattice relaxation time (T1) as a function of the Larmor frequency, predicted by Kimmich and Fatkullin, which were developed from the renormalized Rouse formalism. Three different regions of power laws explaining the relaxation dispersion for the chain mode dynamics of polymer melts can be distinguished and have been observed experimentally over a range of molecular weights and temperatures for several types of polymer melts. In these studies, the T1 relaxation dispersions of polymer melts and networks were obtained over a wide range of frequencies and temperatures employing NMR field cycling relaxometry, and WLF master curves were reconstructed to describe the dispersion over more than nine orders of magnitude. Influences of chemical and physical constraints on the relaxation dispersion of polymers were investigated. This included the introduction of chemical cross-links and filler particles to produce elastomers, for which further constraints were imposed by mechanical deformation. As an alternative means to affect molecular mobility, investigations were also carried out for polystyrene-polybutadiene (PS-PB) copolymers. In all cases, the T1 relaxation dispersions directly reflect the molecular dynamics which were altered in different ways by the inflicted restrictions. Moreover, solvent induced molecular mobility in both polymer melts and networks were studied and additional experiments measuring the transverse relaxation rate and the dipolar coupling strength via double quantum encoding were conducted. The polymer networks used in these studies were natural rubber, butadiene rubber, and polydimethylsiloxane rubber samples. The results obtained here are interpreted based on the molecular dynamics described for the polyisoprene (PI), polybutadiene (PB), and polydimethylsiloxane (PDMS) melts of higher molecular weight (Mw > Mc), where only the physical entanglements are present. In terms of field cycling relaxometry, the power-law relations, which describe the chain mode dynamics for the polymer melts, were used to analyze the experimentally obtained data. The results obtained from the PI melts were somewhat different from those of the other polymers and, therefore, partially deuterated PI melts were investigated employing proton and deuteron experiments to elucidate whether the chemical structure or the intermolecular interaction play the significant role for this particular behavior
Characterization of natural porous media by NMR and MRI techniques : high and low magnetic field studies for estimation of hydraulic properties
No full textSoil is the natural material that covers most of the dry surface of the earth. It is the product of mechanical, chemical and biological interactions of different types of elements. Soils have a very complex composition and high variability in their occurrence and properties. The non-homogeneous mixture and interaction of their components and the changes induced upon usage ensures the complexity of their structure and a comfortable variable spatial distribution over the surface of the planet. The soils components can be found in nature in all the three aggregation states of matter: the solid state, the liquid state and the gas state. In spite of their complex structure and composition, soils can be easily handled and studied by a variety of methods. Prediction of water movement in soils is a controlling factor in various processes of interest in water resources management such as: the runoff generation, the water and nutrients supply to vegetation, the groundwater recharge and contamination. From the physics point of view water distribution and transport in unsaturated soil represents a rather complex problem of porous media hydraulics. Among various techniques developed for investigating hydraulic phenomena in soils, Nuclear Magnetic Resonance (NMR) can be used successfully for the characterization of natural porous media. The aim of this thesis is to apply different NMR techniques for: i) understanding the relaxometric properties of unsaturated natural porous media and ii) for a reliable quantification of water content and its spatial and temporal change in model porous media and soil cores. For that purpose, porous media with increasing complexity and heterogeneity were used (coarse and fine sand and different mixture of sand/clay) to determine the relaxation parameters in order to adapt optimal sequence and parameters for water imaging. Conventional imaging is mostly performed with superconducting high field scanners but low field scanners promise longer relaxation times and therefore smaller loss of signal from water in small and partially filled pores. By this reason high and low field NMR experiments were conducted on these porous media to characterize the dependence on the magnetic field strength. Correlations of the NMR experiments with classical soil physics method like mercury intrusion porosimetry; water retention curves (pF) and multi-step-outflow (MSO) were performed for the characterization of the hydraulic properties of the materials. Due to the extensive research the experiments have been structured in three major parts as follows. In the first part a comparison study between relaxation experiments in high and low magnetic field was performed in order to observe the influence of the magnetic field on the relaxation properties. Due to these results, in the second part of the study only low field relaxation experiments were used in the attempt of correlations with classical soil physics methods (mercury intrusion porosimetry and water retention curves) for characterizing the hydraulic behavior of the samples. Further, the aim was to combine also MRI experiments (2D and 3D NMR) with classical soil physics methods (multi-step-outflow, MSO) for the same purpose of investigating the hydraulic properties. Because low field MRI systems are still under developing for the moment, the MRI experiments were performed in high magnetic field and combined with MSO experiments in order to asses the changes in water content over the depth of the samples during pressure application, changes that are governed by the hydraulic properties of the material. The thesis is structured into 7 chapters. First chapter is an introduction to actual state of NMR research in soil science. The second and the third chapters are theoretical consideration over the NMR and soil physics methods that have been used for investigations. Nevertheless, only the basic theoretical aspects are covered here since due to extensive research we find it more appropriate to introduce each method used, separately, further on, into the content of the thesis. The materials and methods used for investigation differ from one part of the research to another; therefore they will be discussed in detail for each chapter separately. Chapter 4 is the comparison study between high and low field NMR; in chapter 5 is described the combination of low field NMR relaxometry with soil physics methods; chapter 6 presents the MRI studies on a coaxial sample and the parameterization of the water retention functions using a combination of MRI and MSO experiments. In chapter 7 a summary of the entire research is performed and the main conclusions are presented together with an outlook for further researches based on the obtained results
Investigations of aluminophosphate molecular sieves applying noble gas and methane NMR spectroscopy
Get PDFAbstractIn this thesis, nuclear magnetic resonance (NMR) spectroscopy of noble gas atoms and methane molecule is applied to the structure studies of solid microporous materials (molecular sieves). This is an indirect method, where changes to the spectral parameters (i.e. isotropic and anisotropic nuclear shielding tensors) of the adsorbate induced by the investigated environment are measured at variable temperatures and adsorbate loadings. The data are analyzed concentrating on lineshape fitting methods and the correlations with the sieve framework parameters are examined. The studies in this thesis involve resonance signals mostly from 129Xe and 13C nuclei.Molecular sieves, such as zeolites, are typically composed of oxygen, silicon, and aluminium. They are found in the nature, but in most cases they are manufactured synthetically. These materials have a wide variety of industrial applications, such as dehydration, odour and pollutant removal, ion exchange, water softening, etc. The size and shape of molecular-size intracrystallite cavities and channels vary considerably. This thesis focuses on aluminophosphate molecular sieves and their silicon-containing counterparts.In molecular sieve systems, the effects of intra- and intercrystallite motions and exchange of adsorbates on their NMR spectra is discussed. Spectroscopic methods of magic angle spinning and 2D exchange spectroscopy are applied with good results. A technique employing pulsed field gradients to reveal shielding anisotropy component of methane from overlapping resonances from less restricted methane is used. The effects of cations located in molecular sieve frameworks are studied and discussed. From the fast exchange effects on spectral data the intercrystallite cavity sizes are estimated. Spectral differences between circular and elliptical cross-sections of framework channels are also detected. The roles of adsorbate-wall and mutual adsorbate interactions on spectral characteristics are identified and discussed.Academic Dissertation to be presented with the assent of the Faculty of Science, University of Oulu, for public discussion in Auditorium L10 (Raahensali), Linnanmaa, on November 13th, 2004, at 12 noon.Abstract
In this thesis, nuclear magnetic resonance (NMR) spectroscopy of noble gas atoms and methane molecule is applied to the structure studies of solid microporous materials (molecular sieves). This is an indirect method, where changes to the spectral parameters (i.e. isotropic and anisotropic nuclear shielding tensors) of the adsorbate induced by the investigated environment are measured at variable temperatures and adsorbate loadings. The data are analyzed concentrating on lineshape fitting methods and the correlations with the sieve framework parameters are examined. The studies in this thesis involve resonance signals mostly from 129Xe and 13C nuclei.
Molecular sieves, such as zeolites, are typically composed of oxygen, silicon, and aluminium. They are found in the nature, but in most cases they are manufactured synthetically. These materials have a wide variety of industrial applications, such as dehydration, odour and pollutant removal, ion exchange, water softening, etc. The size and shape of molecular-size intracrystallite cavities and channels vary considerably. This thesis focuses on aluminophosphate molecular sieves and their silicon-containing counterparts.
In molecular sieve systems, the effects of intra- and intercrystallite motions and exchange of adsorbates on their NMR spectra is discussed. Spectroscopic methods of magic angle spinning and 2D exchange spectroscopy are applied with good results. A technique employing pulsed field gradients to reveal shielding anisotropy component of methane from overlapping resonances from less restricted methane is used. The effects of cations located in molecular sieve frameworks are studied and discussed. From the fast exchange effects on spectral data the intercrystallite cavity sizes are estimated. Spectral differences between circular and elliptical cross-sections of framework channels are also detected. The roles of adsorbate-wall and mutual adsorbate interactions on spectral characteristics are identified and discussed
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
Ortsaufgelöste Niederfeld-NMR-Untersuchungen der Wasser- und Proteindynamik an Knorpelgewebe sowie dessen Bestandteilen zur Charakterisierung von degenerativen Erkrankungen
Get PDFHyaliner artikulärer Knorpel stellt aufgrund seiner möglichen Einteilung in drei verschiedene Zonen, welche anhand des Kollagenfaserverlaufs vorgenommen werden kann, ein Beispiel eines organisierten biologischen Gewebes dar. Diese tiefenabhängige Struktur wird von einem Gradienten zunehmender Glykosaminoglykan- (Protein-Disacharid-Makromoleküle) und abnehmender Wasserkonzentration zur Knochengrenzfläche hin begleitet. Bei niedrigen Magnetfeldstärken ist die longitudinale Relaxationszeitsvariation (T1) ein zuverlässiger Parameter zur Charakterisierung dieser Struktur. Überdies ist die T1-Zeit sensitiv zum enzymatischen Abbau, externer mechanische Belastung und Degeneration in Folge einer osteoarthritischen Erkrankung. Andererseits ermöglicht die feldabhängige Relaxometrie Zugang zu der quadrupolaren Wechselwirkung. Hierdurch wird der Gehalt und die Ordnung akromolekularer Bestandteile, im Knorpel sind dies die Glykosaminoglykane und Kollagenfasern, widergespiegelt. In diesem Projekt wurden sowohl Niedrigfeld- als auch feldabhängige Techniken kombiniert, um die Wasser- und Proteindynamik von artikulärem Knorpel als auch seinen Bestandteilen zu untersuchen. Erste Versuche fanden an Rinderknorpel und Glykosa-minoglykan / Kollagen - Wasser Modellsystemen statt. Im weiteren Verlauf wurden beide NMR-Techniken zum ersten Mal verwendet, um Korrelationen zwischen NMR-Parametern und dem Verhalten unter mechanischer Belastung bzw. dem Krankheitszustand einer Osteoarthritis bei humanem Knorpel zu identifizieren. Dabei konnte eine Abhängigkeit der Ausprägung der quadrupolaren Wechselwirkung und der Knorpeldicke mit dem Osteoarthritiszustand, welcher durch die Mankin-Grade abgebildet wurde, gefunden werden. Weiterhin ist eine signifikante Korrelation sowohl der Position als auch des maximalen T1-Wertes unter uniaxialer Belastung von 0,6 MPa, einem typischen Werte für eine Druckbelastung im menschlichen Knie, demonstriert wurden. Beide Ergebnisse sind unter Beachtung der räumlichen Auflösung des Tiefenscanners von 50 [my]m, welche eine Größenordnung besser als bei klinisch eingesetzten Geräten ist, von besonderer Wichtigkeit. Zusammenfassend ist festzuhalten, dass Niedrigfeld-NMR-Techniken zusätzliche Informationen zu den Hochfeldergebnissen liefern und damit einen entscheidenden Beitrag zum Gesamtbild des Knorpelgesundheitszustandes beitragen können.Hyaline articular cartilage is an example of a well-organized biological tissue due to the distinction by three layers of differently orientied collagen fibers. Furthermore, its depth-dependent structure is accompanied by a gradient of increasing glycosaminoglycan (protein-dissacharid macromolecules) as well as decreasing water concentration from the top layer towards the bone interface. At low magnetic field strengths, the longitudinal relaxation time (T1) variation within the tissue represents a robust parameter characterizing its layered structure. Moreover, T1 is sensitive to factors such as enzymatic degradation, external load and degeneration, which might be a consequence of an ongoing osteoarthritis disease. Variable-field relaxometry, on the other hand, provides access to the quadrupolar dips. Therefore the content as well as local order of macromolecular constituents, which are glycosaminoglycans and collagen, is reflected. In this project, low-field and variable-field techniques were combined to investigate the water and proteine dynamic in articular cartilage and its constitutes. Therefore first experiments were performed on bovine cartilage and glycosaminoglycan / collagen - water model systems. In a further step, both NMR techniques were combined for the first time to correlate corresponding NMR parameters and the response to load with the severity of osteoarthritis on human cartilage samples. As a result of these experiments the magnitude of the quadrupolar dips as well as the cartilage thickness was found to correlate with osteoarthritis phase measured by its Manking grade. In addition, a significant correlation for the T1 position and its actual value after uniaxial compression at 0.6 MPa, a typical value for forces appearing in the human knee joint, could be demonstrated. This finding is of importance since the spatial resolution of 50 µm obtained with the applied single-sided scanner is about one order of magnitude better than achievable in clinical high-field or low-field scanners. In conclusion, the used low-field techniques could be proven to supply additional information, which can be combined with high-field results, to contribute to the big picture of the cartilage health status
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
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