Artículos de revistas: "Internal filed NMR"

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Bernal, O. O., M. E. Moroz, H. G. Lukefahr, D. E. MacLaughlin, J. A. Mydosh, A. A. Menovsky y C. Ortiz. "Ambient-pressure NMR in URu2Si2: internal-field anisotropy". Journal of Magnetism and Magnetic Materials 272-276 (mayo de 2004): E59—E60. http://dx.doi.org/10.1016/j.jmmm.2003.11.159.

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2

XIE, Ran-Hong y Li-Zhi XIAO. "NMR Logging Probing the Internal Magnetic Field Gradients of Rocks". Chinese Journal of Geophysics 52, n.º 3 (mayo de 2009): 650–56. http://dx.doi.org/10.1002/cjg2.1386.

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3

AKAY, Cengiz y Handan ENGİN KİRİMLİ. "NMR Spectroscopy in the Earth's Magnetic Field". Bitlis Eren Üniversitesi Fen Bilimleri Dergisi 12, n.º 2 (22 de mayo de 2023): 428–34. http://dx.doi.org/10.17798/bitlisfen.1232239.

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Magnetic imaging systems used today are quite expensive and are generally used for medical purposes. Apart from this purpose, there are many scientific fields of study whose internal structure is desired to be displayed. Especially in science, different from the techniques used to understand the internal structure of matter, magnetic imaging techniques are also needed. Therefore, the interest in more useful and smaller magnetic imaging systems is increasing. For this purpose, studies on magnetic particle imaging and magnetic resonance imaging techniques have gained momentum. The magnetic resonance imaging technique, which is one of the magnetic imaging systems based on the NMR phenomenon, has passed through numerous stages and has become smaller and more useful. This study examines the basic components of the NMR images made in the earth's magnetic field for different liquids, the T1 and T2 proton relaxation parameters, and the technique of the obtained two-dimensional images with the EFNMR system.

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4

Isaac-Lam, Meden F. "Determination of Alcohol Content in Alcoholic Beverages Using 45 MHz Benchtop NMR Spectrometer". International Journal of Spectroscopy 2016 (17 de agosto de 2016): 1–8. http://dx.doi.org/10.1155/2016/2526946.

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Alcohol or ethanol is considered the most widely used recreational drug worldwide, and its production, consumption, and sale are strictly regulated by laws. Alcohol content of alcoholic beverages (wine, beers, and spirits) is about 3–50% v/v. Analytical methods to determine the alcohol content must be reliable, precise, and accurate. In this study, the amount of ethanol in several alcoholic beverages was determined using a 45 MHz low-field benchtop NMR (nuclear magnetic resonance) spectrometer. Internal standard and standard addition analytical methods were utilized to quantify ethanol. For both methods, acetic acid or acetonitrile was used as internal standard to quantify alcohol content by using the peak area corresponding to the methyl peaks of ethanol, acetic acid, or acetonitrile. Results showed that internal standard method gave values of percent alcohol that are in close agreement with the indicated label as confirmed by running the samples in a 400 MHz high-field NMR spectrometer using acetic acid as internal standard. This study demonstrates the utility of a benchtop NMR spectrometer that can provide an alternative technique to analyze percent alcohol in alcoholic products.

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Kirkland, Catherine M., Julia R. Krug, Frank J. Vergeldt, Lenno van den Berg, Aldrik H. Velders, Joseph D. Seymour, Sarah L. Codd, Henk Van As y Merle K. de Kreuk. "Characterizing the structure of aerobic granular sludge using ultra-high field magnetic resonance". Water Science and Technology 82, n.º 4 (27 de julio de 2020): 627–39. http://dx.doi.org/10.2166/wst.2020.341.

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Abstract Despite aerobic granular sludge wastewater treatment plants operating around the world, our understanding of internal granule structure and its relation to treatment efficiency remains limited. This can be attributed in part to the drawbacks of time-consuming, labor-intensive, and invasive microscopy protocols which effectively restrict samples sizes and may introduce artefacts. Time-domain nuclear magnetic resonance (NMR) allows non-invasive measurements which describe internal structural features of opaque, complex materials like biofilms. NMR was used to image aerobic granules collected from five full-scale wastewater treatment plants in the Netherlands and United States, as well as laboratory granules and control beads. T1 and T2 relaxation-weighted images reveal heterogeneous structures that include high- and low-density biofilm regions, water-like voids, and solid-like inclusions. Channels larger than approximately 50 μm and connected to the bulk fluid were not visible. Both cluster and ring-like structures were observed with each granule source having a characteristic structural type. These structures, and their NMR relaxation behavior, were stable over several months of storage. These observations reveal the complex structures within aerobic granules from a range of sources and highlight the need for non-invasive characterization methods like NMR to be applied in the ongoing effort to correlate structure and function.

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6

Nestle, Nikolaus, Asal Qadan, Petrik Galvosas, Wolfgang Süss y Jörg Kärger. "PFG NMR and internal magnetic field gradients in plant-based materials". Magnetic Resonance Imaging 20, n.º 7 (septiembre de 2002): 567–73. http://dx.doi.org/10.1016/s0730-725x(02)00529-5.

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7

Johnson, Andrew y Hugh Daigle. "Nuclear magnetic resonance secular relaxation measurements as a method of extracting internal magnetic field gradients and pore sizes". Interpretation 4, n.º 4 (1 de noviembre de 2016): T557—T565. http://dx.doi.org/10.1190/int-2015-0127.1.

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Nuclear magnetic resonance (NMR) has been used as a common and powerful tool for petrophysical investigation of fluid-bearing porous media. A major complication of NMR analysis occurs, however, when diffusion of fluid protons through magnetic field heterogeneities becomes nonnegligible. A quantity called the secular relaxation rate ([Formula: see text]) has been defined as the difference in transverse and longitudinal relaxation rates ([Formula: see text]-[Formula: see text]) and can be shown to isolate the effects of diffusion as a function of pore system parameters. We have developed results that extract internal magnetic field gradient strengths based on changes in [Formula: see text] as a function of the NMR interecho spacing. We also indicated that an optimization algorithm can be used to invert for volumetrically weighted mean pore sizes. The benefit of these types of analyses is to provide simple methodologies for inferring the average strengths of internal magnetic field gradients and pore sizes from NMR measurements without the need for independent measurements of pore size, such as from mercury injection porosimetry. In addition, secular relaxation analysis removes complicating effects provided by bulk fluid and other nondiffusion relaxation mechanisms.

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8

Boddenberg, B. y G. Neue. "Spin I = 1 NMR Patterns for the Case of Twofold Internal Spin Interactions". Zeitschrift für Naturforschung A 42, n.º 9 (1 de septiembre de 1987): 948–56. http://dx.doi.org/10.1515/zna-1987-0907.

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An operator formalism is developed which allows a convenient calculation of high field spin I = 1 solid state NMR powder spectra for the case that the spins are acted on not only electrically via an electric field gradient tensor but also magnetically via a shielding tensor. Examples of practical applications in the field of surface science are given.

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9

Brox, Timothy I., Mark L. Skidmore y Jennifer R. Brown. "Characterizing the internal structure of laboratory ice samples with nuclear magnetic resonance". Journal of Glaciology 61, n.º 225 (2015): 55–64. http://dx.doi.org/10.3189/2015jog14j133.

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AbstractDue to solute impurities and freezing-point depression in polycrystalline ice, a complicated and dynamic network of liquid water forms within the solid ice matrix at the boundaries between ice crystal grains. Impurity concentrations, temperature and pressure influence this network structure and impact physical, transport and rheological properties of ice. However, the nature of this internal network structure is not fully understood. Here we utilize nuclear magnetic resonance (NMR) measurements of diffusion and magnetic relaxation to study the geometry and interconnectivity of the liquid-filled network in laboratory ice, formed from a 7 g L−1 NaCl solution, and its evolution due to recrystallization processes. Additionally, we apply these NMR measurements to observe the impact on ice microstructure of an ice-binding protein (IBP) excreted by the V3519-10 organism (Flavobacteriaceae family) isolated from the Vostok ice core in Antarctica. Recrystallization inhibition was observed as a function of IBP concentration. This work demonstrates the utility of advanced NMR techniques for applications to ice microstructure and has broader implications for understanding geophysical properties of cryospheric systems.

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10

Mohnke, O. y N. Klitzsch. "Microscale Simulations of NMR Relaxation in Porous Media Considering Internal Field Gradients". Vadose Zone Journal 9, n.º 4 (noviembre de 2010): 846–57. http://dx.doi.org/10.2136/vzj2009.0161.

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11

Andreev, A. S., O. B. Lapina y S. V. Cherepanova. "A New Insight into Cobalt Metal Powder Internal Field 59Co NMR Spectra". Applied Magnetic Resonance 45, n.º 10 (28 de agosto de 2014): 1009–17. http://dx.doi.org/10.1007/s00723-014-0580-0.

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12

Yakovlev, Ilya V., Stanislav S. Yakushkin, Mariya A. Kazakova, Sergey N. Trukhan, Zoya N. Volkova, Alexander P. Gerashchenko, Andrey S. Andreev et al. "Superparamagnetic behaviour of metallic Co nanoparticles according to variable temperature magnetic resonance". Physical Chemistry Chemical Physics 23, n.º 4 (2021): 2723–30. http://dx.doi.org/10.1039/d0cp05963c.

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13

Weigler, Max, Edda Winter, Benjamin Kresse, Martin Brodrecht, Gerd Buntkowsky y Michael Vogel. "Static field gradient NMR studies of water diffusion in mesoporous silica". Physical Chemistry Chemical Physics 22, n.º 25 (2020): 13989–98. http://dx.doi.org/10.1039/d0cp01290d.

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14

Daigle, Hugh, Andrew Johnson y Brittney Thomas. "Determining fractal dimension from nuclear magnetic resonance data in rocks with internal magnetic field gradients". GEOPHYSICS 79, n.º 6 (1 de noviembre de 2014): D425—D431. http://dx.doi.org/10.1190/geo2014-0325.1.

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Pore size distributions in rocks may be represented by fractal scaling, and fractal descriptions of pore systems may be used for prediction of petrophysical properties such as permeability, tortuosity, diffusivity, and electrical conductivity. Transverse relaxation time ([Formula: see text]) distributions determined by nuclear magnetic resonance (NMR) measurements may be used to determine the fractal scaling of the pore system, but the analysis is complicated when internal magnetic field gradients at the pore scale are sufficiently large. Through computations in ideal porous media and laboratory measurements of glass beads and sediment samples, we found that the effect of internal magnetic field gradients was most pronounced in rocks with larger pores and a high magnetic susceptibility contrast between the pore fluid and mineral grains. We quantified this behavior in terms of pore size and Carr-Purcell-Meiboom-Gill (CPMG) half-echo spacing through scaling arguments. We additionally found that the effects of internal field gradients may be mitigated in the laboratory by performing [Formula: see text] measurements with different CPMG half-echo spacings and fitting the apparent fractal dimensions determined by the NMR measurements with a model to determine the true pore system fractal dimension.

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15

Walbrecker, Jan O., Rosemary Knight y Elliot Grunewald. "The impact of prepolarization on Earth’s field laboratory nuclear-magnetic-resonance relaxation experiments". GEOPHYSICS 79, n.º 3 (1 de mayo de 2014): EN39—EN48. http://dx.doi.org/10.1190/geo2013-0272.1.

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To advance our understanding, and the application of surface nuclear magnetic resonance (NMR), we need to conduct Earth’s magnetic field (EF) laboratory NMR measurements of the acquired relaxation times. In EF laboratory NMR, prepolarization is used to achieve detectable signal levels. This involves exposing the sample to a static magnetic field to increase the equilibrium magnetization associated with the nuclear spins of protons in the pore fluid. We found that prepolarization can also have a significant impact on the commonly measured relaxation time [Formula: see text]. We studied this impact on [Formula: see text] using a set of sand samples taken from boreholes in the U.S. High Plains aquifer. Using rock-magnetic measurements, we found that prepolarization at 25 mT increased the magnetization of the solid phase of the samples up to a factor of 10. For these samples, we observed [Formula: see text] that decreased from 30 to 3 ms with increasing magnetization. Surface NMR data collected at the borehole site indicate significantly longer [Formula: see text] (50–80 ms) at the depths from which the samples were taken. We attributed our observations to prepolarization inducing remanent magnetization in the solid phase of the sample, which results in elevated internal magnetic fields that reduce [Formula: see text]. In contrast, we found good agreement between EF laboratory and surface NMR measurements of the relaxation time [Formula: see text]. Because [Formula: see text] is unaffected by internal fields, this supports our conclusion that prepolarization is significantly impacting the laboratory measurement of [Formula: see text]. To mitigate these undesired effects, we have developed the inclusion of a demagnetization pulse after prepolarization in EF laboratory experiments. By applying the demagnetization pulse along the same direction of the static field only the remanent magnetization of the solid phase of the sample would be removed, whereas the prepolarized state of the nuclear spin magnetization of the liquid phase would be retained.

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16

Zhang, Yan, Lizhi Xiao, Guangzhi Liao y Bernhard Blümich. "Direct correlation of internal gradients and pore size distributions with low field NMR". Journal of Magnetic Resonance 267 (junio de 2016): 37–42. http://dx.doi.org/10.1016/j.jmr.2016.04.009.

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17

Hanna, George M. y Cesar A. Lau-Cam. "Simple HNMR Spectroscopic Method for Assay of Salts of the Contrast Agent Diatrizoate in Commercial Solutions". Journal of AOAC INTERNATIONAL 79, n.º 4 (1 de julio de 1996): 833–38. http://dx.doi.org/10.1093/jaoac/79.4.833.

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Abstract A simple, accurate, and specific 1H NMR spectroscopic method was developed for the assay of diatrizoate meglumine or the combination diatrizoate meglumine and diatrizoate sodium in commercial solutions for injection. A mixture of injectable solution and sodium acetate, the internal standard, was diluted with D20 and the 1H NMR spectrum of the solution was obtained. Two approaches were used to calculate the drug content, based on the integral values for the -N-CO-CH3 protons of diatrizoic acid at 2.23 ppm, the N-CH3 protons of meglumine at 2.73 ppm, and the CH3-CO-protons of sodium acetate at 1.9 ppm. Recoveries (mean ± standard deviation) of diatrizoic acid and meglumine from 10 synthetic mixtures of various amounts of these compounds with a fixed amount of internal standard were 100.3 ±; 0.55% and 100.1 ± 0.98%, respectively. In addition to providing a direct means of simultaneously assaying diatrizoic acid and meglumine, the proposed NMR method can also be used to identify diatrizoate meglumine and each of its molecular components.

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18

Vasyukova, Anna, Konstantin Krivoshonok, Marina Vedenyapina, Vladimir Kuznetsov y B. Tverdokhleb. "FORMATION OF THE TASTE OF COMBINED MINCED FISH IN THE PROCESS OF CULINARY PROCESSING". Fisheries 2022, n.º 3 (14 de junio de 2022): 99–103. http://dx.doi.org/10.37663/0131-6184-2022-3-99-103.

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The purpose of the study is to form the taste of combined minced fish and ready-made culinary products. Low-field nuclear magnetic resonance (1H-NMR) have been accepted widely as a non-destructive analytical technique in food processing technology to their sensitivity, non-invasiveness, rapidness, and cost-effectiveness. Moreover, the ability to provide real-time information on products during and after processing has been linked to the use of thice analytical technique. Timely information on quality parameters in food processing provided by online monitoring using 1H-NMR may increase the quality of the product, improve operation process, and enhance production economy in food field. In this review, the use of online 1H-NMR in food processing techniques, such as freezing, frying, fermentation, and internal quality analysis, is explored. Limitations and need for further development are outlined.

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19

Davies, Antony N. y Peter Lampen. "JCAMP-DX for NMR". Applied Spectroscopy 47, n.º 8 (agosto de 1993): 1093–99. http://dx.doi.org/10.1366/0003702934067874.

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Following the development and publication of the JCAMP-DX protocol 4.24 and its successful implementation in the field of infrared spectroscopy, data exchange without loss of information, between systems of different origin and internal format, has become a reality. The benefits of this system-independent data transfer standard have been recognized by workers in other areas who have expressed a wish for an equivalent, compatible standard in their own fields. This publication details a protocol for the exchange of Nuclear Magnetic Resonance (NMR) spectral data without any loss of information and in a format that is compatible with all storage media and computer systems. The protocol detailed below is designed for spectral data transfer, and its use for NMR imaging data transfer has not as yet been investigated.

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20

Schaefer, Ted, Christian Beaulieu y Rudy Sebastian. "2-Phenyladamantane as a model for axial phenylcyclohexane. 1H NMR and molecular orbital studies of motion about the Csp2—Csp3 bond". Canadian Journal of Chemistry 69, n.º 3 (1 de marzo de 1991): 503–8. http://dx.doi.org/10.1139/v91-075.

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The 1H NMR spectra of the aromatic groups of 2-phenylcyclohexane and 2-phenyladamantane, in CS2/C6D12 solution at 300 K, are analyzed to yield the long-range coupling constants between the α and ring protons. The coupling over six bonds is related to the internal rotational potential about the Csp2—Csp3 bond in these molecules. It is confirmed that the equatorial isomer of phenylcyclohexane has the parallel conformer, that in which the aromatic plane lies in the symmetry plane bisecting the cyclohexane moiety, as the most stable. The apparent twofold barrier to rotation about the exocyclic carbon–carbon bond follows as 7.1 kJ/mol from the six-bond coupling constant. For 2-phenyladamantane, the six-bond coupling constant strongly implies that the perpendicular conformer, perhaps slightly skewed, is that of lowest energy and that the apparent twofold barrier to rotation about the Csp2—Csp3 bond is about 7.5 kJ/mol. Insofar as 2-phenyladamantane mimics axial phenylcyclohexane, these results confirm recent conclusions about the conformation of the latter and provide evidence for its internal mobility. Geometry-optimized AMI and STO-3G MO computations are reported for the internal motion in both isomers of phenylcyclohexane. The former agree best with experiment for the equatorial isomer, but both imply a significant fourfold, of opposite sign to the twofold, component of the internal rotational potential. For the axial isomer, the two sets of computations find a skewed perpendicular conformer as most stable, in rough agreement with force-field results. However, the barrier to rotation about the Csp2—Csp3 bond is computed as small and AMI has the parallel conformer as more stable than the perpendicular. Key words: 2-phenyladamantane, 1H NMR and internal rotation; phenylcyclohexane, 1H NMR and internal rotation; MO computations, 2-phenyladamantane and phenylcyclohexane.

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21

Herbert-Pucheta, José Enrique, Álvaro Omar Hernández-Rangel, María Elena Vargas-Díaz, Karla Hernández Sánchez, Luis Gerardo Zepeda-Vallejo y Montserrat Jiménez-García. "Silicone-specific identification of trace polydimethylsiloxanes in wines with 2D-diffusion-ordered nuclear magnetic resonance spectroscopy (DOSY-NMR)". BIO Web of Conferences 68 (2023): 02004. http://dx.doi.org/10.1051/bioconf/20236802004.

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Present work stresses a novel analytical approach for increasing the specificity of standard NMR approaches for identifying polydimethylsiloxane (PDMS) and further silicone moieties in wines’ organic extracts, by including a second dimension that correlates chemical shifts with diffusion coefficients by means of pulsed-field gradient diffusion ordered spectroscopy (DOSY-NMR). Each silicone source in wines is unambiguously assigned by correlation of both local chemical environments and by a unique diffusion coefficient value, in turn related to a hydrodynamic radius (RH) that can be obtained with respect proper internal standards. Obtained PDMS diffusion coefficient values and hydrodynamic radii in wines’ extracts, in agreement with expected values, present a selectivity and specificity so far not reported, that positions DOSY-NMR spectroscopy as an alternative in oenology for controlling PDMS limits.

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22

Chen, Yannan, Hongjing Dong, Jingkun Li, Lanping Guo y Xiao Wang. "Evaluation of a Nondestructive NMR and MRI Method for Monitoring the Drying Process of Gastrodia elata Blume". Molecules 24, n.º 2 (10 de enero de 2019): 236. http://dx.doi.org/10.3390/molecules24020236.

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Gastrodia elata Blume (G. elata) is a prominent traditional herb and its dry tuber is officially listed in the Chinese Pharmacopoeia. To ensure the quality of dried G. elata, the establishment of a nondestructive and convenient method to monitor the drying process is necessary. In this study, a nondestructive low-field nuclear magnetic resonance (LF-NMR) and magnetic resonance imaging (MRI) method was introduced to monitor the drying process of G. elata. Three water states (bound, immobilized, and free) in G. elata samples were investigated through multiexponential fitting and inversion of the NMR data. The variation and distribution of the three water states during drying were monitored by LF-NMR, and the spatial distribution of water and internal structural changes were analyzed by MRI. Linear analysis of the moisture content, L* (lightness), b* (yellowness), and NMR parameters showed good correlations among them. Furthermore, partial least squares regression (PLSR) model analysis, which takes into account all NMR parameters, also showed good correlations among these parameters. All results showed that LF-NMR was feasible and convenient for monitoring moisture content. Therefore, LF-NMR and MRI could be used to monitor the moisture content nondestructively in the drying process of Chinese traditional herbs.

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23

Magishi, K., M. Kawakami, T. Saito, K. Koyama, K. Mizuno y S. Kunii. "11B NMR Study of CexLa1–xB6". Zeitschrift für Naturforschung A 57, n.º 6-7 (1 de julio de 2002): 441–46. http://dx.doi.org/10.1515/zna-2002-6-727.

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We have carried out 11B NMR experiments on single crystals of CexLa1-xB6 in order to investigate the nature of phase IV. The NMR spectrum undergoes an appreciable broadening by the internal magnetic field as is lowered in phase IV, and the nuclear spin-lattice relaxation rate, 1/T1, exhibits a sharp peak around the phase I-IV boundary. Also, in phase III the amplitude of the antiferromagnetic (AFM) moment is large enough even just below the phase IV-III transition, which suggests that the AFM moment grows considerably in phase IV. These results support the view that phase IV is an AFM ordered phase

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24

Keener, Kevin M., Richard L. Stroshine y John A. Nyenhuis. "Evaluation of Low Field (5.40-MHz) Proton Magnetic Resonance Measurements of Dw and T2 as Methods of Nondestructive Quality Evaluation of Apples". Journal of the American Society for Horticultural Science 124, n.º 3 (mayo de 1999): 289–95. http://dx.doi.org/10.21273/jashs.124.3.289.

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A 5.40-MHz NMR system was used for measuring the self-diffusion coefficient of water (Dw) and the spin-spin relaxation constant (T2) in apple (Malus ×domestica Borkh.) tissue. The pulsed field gradient spin echo (PFGSE) technique was used to measure Dw, and the Carr-Purcell-Meiboom-Gill (CPMG) technique was used to measure T2. T2 and Dw values were compared for apples with differing amounts of soluble solids concentration (SSC) and with and without internal defects, such as bruising, watercore, and internal browning. `Granny Smith', `Golden Delicious', and `Delicious' apples were tested. In `Golden Delicious', Dw highly correlated with apple tissue SSC (P < 0.002, r2 = 0.68). This indicates that Dw could potentially be used for sorting `Golden Delicious” apples based on SSC, but the coefficient of determination needs to be improved before it would be commercially viable. There were no measurable differences in Dw among healthy apple tissue and tissue affected by either watercore or internal browning. T2 values showed no relationship between healthy apple tissue and bruised tissue in `Golden Delicious' and `Granny Smith'. However, in `Delicious' tissue, T2 values were statistically different between healthy and bruised tissue (P < 0.02). Further comparisons in `Delicious' between watercore and healthy apple tissue showed no differences. But, there were statistical differences found between T2 in healthy apple tissue and tissue with internal browning (P < 0.01). These results indicate that T2 could potentially be used for separating `Delicious' apples with internal browning or with bruising from healthy apples. Titratable acids and pH were correlated for `Golden Delicious' (P < 0.08). This correlation is significant because one may be able to noninvasively measure pH in `Golden Delicious' apples using NMR, which could then be correlated to titratable acids.

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Pogoreltsev, A. I., A. N. Gavrilenko, V. L. Matukhin, B. V. Korzun y E. V. Schmidt. "Electron-density distribution in CuFeS2 as determined by 63,65Cu NMR in an internal magnetic field". Journal of Applied Spectroscopy 80, n.º 3 (julio de 2013): 351–56. http://dx.doi.org/10.1007/s10812-013-9772-x.

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26

Faes, Matthias y David Moens. "On auto‐ and cross‐interdependence in interval field finite element analysis". International Journal for Numerical Methods in Engineering 121, n.º 9 (15 de mayo de 2020): 2033–50. http://dx.doi.org/10.1002/nme.6297.

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Morris, P. G. "Sir Peter Mansfield. 9 October 1933—8 February 2017". Biographical Memoirs of Fellows of the Royal Society 70 (3 de febrero de 2021): 313–34. http://dx.doi.org/10.1098/rsbm.2020.0031.

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Peter Mansfield's rise from humble origins to founding father of magnetic resonance imaging (MRI) is an inspirational and remarkable story. His first scientific contributions were in the field of solid state nuclear magnetic resonance (NMR), and it was whilst trying to develop an NMR version of X-ray crystallography that he developed the underpinning methodology for MRI. At that time (the early 1970s) NMR was an analytical tool, ubiquitous in chemistry departments. For most of those working in the field, there was no hint that it could be developed into a diagnostic imaging technique that would reveal internal anatomy in unprecedented detail. Yet that was what happened in the space of just a few years. The first MRI scans were slow, and Peter was driven to speed them up, making physiological and later functional brain imaging studies possible. The technical challenges were many, and eschewed by healthcare equipment providers, but Peter persisted and his brainchild, echo-planar imaging, came to dominate the high speed MRI field. Peter was a gifted physicist and archetypal inventor who devoted his life to the development of a technique that has saved millions of lives. In 2003, he shared the Nobel Prize for Physiology or Medicine, in recognition of his achievement.

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Reddy, G. Srinivasa, M. Manjunatha y K. P. Ramesh. "59Co Internal field NMR analysis of Co35Fe35Ni30 alloy synthesized via novel low cost chemical reduction technique". Journal of Physics and Chemistry of Solids 148 (enero de 2021): 109703. http://dx.doi.org/10.1016/j.jpcs.2020.109703.

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29

Xing, Dong-hui, Yi-ren Fan, Shao-gui Deng, Xin-min Ge, Jian-yu Liu y Fei Wu. "Influential Factors of Internal Magnetic Field Gradient in Reservoir Rock and Its Effects on NMR Response". Applied Magnetic Resonance 49, n.º 3 (15 de enero de 2018): 227–37. http://dx.doi.org/10.1007/s00723-018-0979-0.

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M, Manjunatha, G. Srinivas Reddy, K. J. Mallikarjunaiah y K. P. Ramesh. "Effect of aluminium substitution in magnetically affluent inverse spinel ferrites studied via 57Fe-Internal field NMR". Journal of Molecular Structure 1209 (junio de 2020): 127956. http://dx.doi.org/10.1016/j.molstruc.2020.127956.

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31

Henrique Novotny, Etelvino, Eduardo Ribeiro deAzevedo, Gustavo de Godoy, Daniel Martelozo Consalter y Miguel Cooper. "Determination of soil pore size distribution and water retention curve by internal magnetic field modulation at low field 1H NMR". Geoderma 431 (marzo de 2023): 116363. http://dx.doi.org/10.1016/j.geoderma.2023.116363.

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32

Nardelli, Francesca, Silvia Borsacchi, Lucia Calucci, Elisa Carignani, Francesca Martini y Marco Geppi. "Anisotropy and NMR spectroscopy". Rendiconti Lincei. Scienze Fisiche e Naturali 31, n.º 4 (16 de agosto de 2020): 999–1010. http://dx.doi.org/10.1007/s12210-020-00945-3.

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Abstract In this paper, different aspects concerning anisotropy in Nuclear Magnetic Resonance (NMR) spectroscopy have been reviewed. In particular, the relevant theory has been presented, showing how anisotropy stems from the dependence of internal nuclear spin interactions on the molecular orientation with respect to the external magnetic field direction. The consequences of anisotropy in the use of NMR spectroscopy have been critically discussed: on one side, the availability of very detailed structural and dynamic information, and on the other side, the loss of spectral resolution. The experiments used to measure the anisotropic properties in solid and soft materials, where, in contrast to liquids, such properties are not averaged out by the molecular tumbling, have been described. Such experiments can be based either on static low-resolution techniques or on one- and two-dimensional pulse sequences exploiting Magic Angle Spinning (MAS). Examples of applications of NMR spectroscopy have been shown, which exploit anisotropy to obtain important physico-chemical information on several categories of systems, including pharmaceuticals, inorganic materials, polymers, liquid crystals, and self-assembling amphiphiles in water. Solid-state NMR spectroscopy can be considered, nowadays, one of the most powerful characterization techniques for all kinds of solid, either amorphous or crystalline, and semi-solid systems for the obtainment of both structural and dynamic properties on a molecular and supra-molecular scale. Graphic abstract

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33

Yue, Yunrui, Song Li, Xinbing Cheng, Jinhong Wei, Fanzheng Zeng, Xiang Zhou y Jianhua Yang. "Effects of temperature on the ohmic internal resistance and energy loss of Lithium-ion batteries under millisecond pulse discharge". Journal of Physics: Conference Series 2301, n.º 1 (1 de julio de 2022): 012014. http://dx.doi.org/10.1088/1742-6596/2301/1/012014.

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Abstract Battery energy storage technology has a promising future in the field of compact high-power pulse drivers due to its high energy storage density. In this paper, three types of high-performance lithium batteries, such as lithium titanate (LTO) battery, lithium iron phosphate (LFP) battery, and Ni,Co,Al (NCR) ternary lithium-ion battery, have been studied in different ambient temperatures by using DC internal resistance measurement method. The result shows that the ohmic internal resistance of lithium batteries increases when the temperature drops. When the temperature is above -30 °C, the ohmic internal resistances of the three types of battery are nearly identical. At -50 °C, the ohmic internal resistances of LFP battery and LTO battery are about 83 mΩ and 151 mΩ, respectively, and the ohmic internal resistance of NCR li-ion battery increases by one order of magnitude. The energy loss from LTO battery and LFP battery during pulse discharge is similar. In the temperature range of -30 °C to 50 °C, the energy loss of NCR li-ion battery pulse discharge is the largest. At -50 °C, the energy loss of NCR li-ion battery is the least.

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34

Cutillo, A. G., K. Ganesan, D. C. Ailion, A. H. Morris, C. H. Durney, S. C. Symko y R. A. Christman. "Alveolar air-tissue interface and nuclear magnetic resonance behavior of lung". Journal of Applied Physiology 70, n.º 5 (1 de mayo de 1991): 2145–54. http://dx.doi.org/10.1152/jappl.1991.70.5.2145.

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Inflated lungs are characterized by a short nuclear magnetic resonance (NMR) free induction decay (rapid disappearance of NMR signal), likely due to internal (tissue-induced) magnetic field inhomogeneity produced by the alveolar air-tissue interface. This phenomenon can also be detected using temporally symmetric and asymmetric NMR spin-echo sequences; these sequences generate a pair of NMR images from which a difference signal (delta) is obtained (reflecting the signal from lung water experiencing the air-tissue interface effect). We measured delta in normal excised rat lungs at inflation pressures of 0-30 cmH2O for asymmetry times (a) of 1-6 ms. Delta was low in degassed lungs and increased markedly with alveolar opening when measured at a = 6 ms (delta 6 ms); delta 6 ms varied little during the rest of the inflation-deflation cycle. Delta 1 ms (a = 1 ms) did not vary significantly on inflation and deflation. Measurements of delta at a = 3 and 5 ms generally lay between those of delta 1 ms and delta 6 ms. These findings, which are consistent with theoretical predictions, suggest that measurements of delta at appropriate asymmetry times are particularly sensitive to alveolar opening and may provide a means of distinguishing alveolar recruitment from alveolar distension in the pressure-volume behavior of the lung.

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35

Li, Na, Yong Shen, Wenru Liu, Jun Mei y Jing Xie. "Low-Field NMR and MRI to Analyze the Effect of Edible Coating Incorporated with MAP on Qualities of Half-Smooth Tongue Sole (Cynoglossus Semilaevis Günther) Fillets during Refrigerated Storage". Applied Sciences 8, n.º 8 (17 de agosto de 2018): 1391. http://dx.doi.org/10.3390/app8081391.

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Nondestructive and fast measurement and characterization of fish is highly desired during various processing treatments. This research investigated the effectiveness of low field LF-NMR and MRI as fast monitoring techniques to estimate the qualities of half-smooth tongue sole fillets treated with edible coating combined with modified atmosphere packaging during refrigeration. T2 relaxation spectra showed three peaks representing bound water (T21), immobile water (T22), and free water (T23), respectively. pT22 accounted for the largest proportion of three types of water, followed by pT23. The weighted MRI provided the internal structure information associated with different samples, indicting the combination of edible coating and MAP (70% CO2 + 30% N2) is the best performance in the maintenance of qualities and freshness of HTS fillets. All results demonstrated that the combination of LF-NMR and MRI as fast and nondestructive methods have great potential to monitor qualities deterioration and predict shelf life in of HTS fillets during refrigerated storage.

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36

Connolly, Paul R. J., Weichao Yan, Daniel Zhang, Mohamed Mahmoud, Michael Verrall, Maxim Lebedev, Stefan Iglauer, Peter J. Metaxas, Eric F. May y Michael L. Johns. "Simulation and experimental measurements of internal magnetic field gradients and NMR transverse relaxation times (T2) in sandstone rocks". Journal of Petroleum Science and Engineering 175 (abril de 2019): 985–97. http://dx.doi.org/10.1016/j.petrol.2019.01.036.

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37

Kärger, J., H. Pfeifer y S. Rudtsch. "The influence of internal magnetic field gradients on NMR self-diffusion measurements of molecules adsorbed on microporous crystallites". Journal of Magnetic Resonance (1969) 85, n.º 2 (noviembre de 1989): 381–87. http://dx.doi.org/10.1016/0022-2364(89)90150-9.

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38

Choudhary, Harish K., M. Manjunatha, R. Damle, K. P. Ramesh y B. Sahoo. "Solvent dependent morphology and 59Co internal field NMR study of Co-aggregates synthesized by a wet chemical method". Physical Chemistry Chemical Physics 20, n.º 26 (2018): 17739–50. http://dx.doi.org/10.1039/c8cp01780h.

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39

Smith, Albert A., Nicolas Bolik-Coulon, Matthias Ernst, Beat H. Meier y Fabien Ferrage. "How wide is the window opened by high-resolution relaxometry on the internal dynamics of proteins in solution?" Journal of Biomolecular NMR 75, n.º 2-3 (marzo de 2021): 119–31. http://dx.doi.org/10.1007/s10858-021-00361-1.

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AbstractThe dynamics of molecules in solution is usually quantified by the determination of timescale-specific amplitudes of motions. High-resolution nuclear magnetic resonance (NMR) relaxometry experiments—where the sample is transferred to low fields for longitudinal (T1) relaxation, and back to high field for detection with residue-specific resolution—seeks to increase the ability to distinguish the contributions from motion on timescales slower than a few nanoseconds. However, tumbling of a molecule in solution masks some of these motions. Therefore, we investigate to what extent relaxometry improves timescale resolution, using the “detector” analysis of dynamics. Here, we demonstrate improvements in the characterization of internal dynamics of methyl-bearing side chains by carbon-13 relaxometry in the small protein ubiquitin. We show that relaxometry data leads to better information about nanosecond motions as compared to high-field relaxation data only. Our calculations show that gains from relaxometry are greater with increasing correlation time of rotational diffusion.

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40

Käyhkö, Jari, Eero Hiltunen, Yrjö Hiltunen, Ekaterina Nikolskaya, Lauri Kulmala y Thaddeus Maloney. "Effect of compression refining on fiber properties". BioResources 15, n.º 4 (25 de septiembre de 2020): 8696–707. http://dx.doi.org/10.15376/biores.15.4.8696-8707.

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This article shows how fiber properties obtained by the compression refining of bleached softwood pulp refined using a KID 300 refiner differs from traditional bar refining. A KID refiner is a stone crusher that has been modified to refine fiber, and it offers a refining method that could be used at the mill scale. This study showed that compression refining caused more internal fibrillation compared with blade refining and improved the pulp’s ability to be beaten. Net energy consumption in compression refining was less than that of bar refining. Compression refining yielded pulp with shorter fibers and a higher number of fines, kinks, and curves. Still, the strength properties of the paper were the same level as bar-refined pulp, probably due to the higher internal fibrillation and flexibility of the fibers. It was also shown that the low field time-domain nuclear magnetic resonance (TD-NMR) method was capable of measuring the porosity and internal fibrillation of the fiber.

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41

Keating, Kristina y Rosemary Knight. "A laboratory study to determine the effect of iron oxides on proton NMR measurements". GEOPHYSICS 72, n.º 1 (enero de 2007): E27—E32. http://dx.doi.org/10.1190/1.2399445.

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Using laboratory methods, we investigate the effect of the presence and mineralogic form of iron on measured proton nuclear magnetic resonance (NMR) relaxation rates. Five samples of quartz sand were coated with ferrihydrite, goethite, hematite, lepidocrocite, and magnetite. The relaxation rates for these iron-oxide-coated sands saturated with water were measured and compared to the relaxation rate of quartz sand saturated with water. We found that the presence of the iron oxides led to increases in the relaxation rates by increasing the surface relaxation rate. The magnitude of the surface relaxation rate was different for the various iron-oxide minerals because of changes in both the surface-area-to-volume ratio of the pore space, and the surface relaxivity. The relaxation rate of the magnetite-coated sand was further increased because of internal magnetic field gradients caused by the presence of magnetite. We conclude that both the concentration and mineralogical form of iron can have a significant impact on NMR relaxation behavior.

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42

Schaefer, Ted, Scott Kroeker y David M. McKinnon. "1H and 13C nuclear magnetic resonance and molecular orbital studies of the internal rotational potential and of spin–spin coupling transmission in phenylallene". Canadian Journal of Chemistry 73, n.º 9 (1 de septiembre de 1995): 1478–87. http://dx.doi.org/10.1139/v95-183.

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The 1H nuclear magnetic resonance spectra of phenylallene, diluted in acetone-d6 and benzene-d6, yield long-range coupling constants over as many as eight formal bonds between the ring and side-chain protons. These are discussed in terms of σ- and π-electron spin–spin coupling mechanisms, which are sensitive to the torsion angle between the allenyl and phenyl fragments. The torsion angle is assessed by means of molecular orbital computations of the internal rotational potential, whose height is calculated as 16.0 kJ/mol at the MP2/6-31G* level of correlation-gradient theory. Comparison with experimental and theoretical internal rotational potentials for styrene suggests that steric repulsions in the planar form of styrene amount to about 4 kJ/mol. In a field of 7.0 T, phenylallene is partially aligned, entailing a positive dipolar coupling constant between the methylene protons, from which absolute signs of the spin–spin coupling constants involving these protons can be inferred. Such coupling constants over seven and eight bonds, to the meta and para protons, are taken as being mediated by the extended π-electron system, providing a measure of π-electron contributions to coupling constants between meta protons and those in side chains (spin correlation). Some coupling constants between protons and 13C nuclei in the side chain, as well as between ring protons and these 13C nuclei, are also discussed in terms of spin coupling mechanisms. Solvent perturbations of one-bond proton–carbon coupling constants in the allenyl group do not follow the usual pattern in which an increase in polarity of the solvent is associated with an increase in the magnitude of the coupling constant. Keywords: 1H NMR, phenylallene; 1H NMR, long-range spin–spin coupling constants in phenylallene; phenylallene, internal rotational potential, molecular orbital computations; molecular orbital calculations, an internal rotational potential in phenylallene.

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43

Farès, Christophe, Julia B. Lingnau, Cornelia Wirtz y Ulrich Sternberg. "Conformational Investigations in Flexible Molecules Using Orientational NMR Constraints in Combination with 3J-Couplings and NOE Distances". Molecules 24, n.º 23 (3 de diciembre de 2019): 4417. http://dx.doi.org/10.3390/molecules24234417.

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The downscaling of NMR tensorial interactions, such as dipolar couplings, from tens of kilohertz to a few hertz in low-order media is the result of dynamics spanning several orders of magnitudes, including vibrational modes (~ns-fs), whole-molecule reorientation (~ns) and higher barrier internal conformational exchange (<ms). In this work, we propose to employ these dynamically averaged interactions to drive an “alignment-tensor-free” molecular dynamic simulation with orientation constraints (MDOC) in order to efficiently access the conformational space sampled by flexible small molecules such as natural products. Key to this approach is the application of tensorial pseudo-force restraints which simultaneously guide the overall reorientation and conformational fluctuations based on defined memory function over the running trajectory. With the molecular mechanics force-field, which includes bond polarization theory (BPT), and complemented with other available NMR parameters such as NOEs and scalar J-couplings, MDOC efficiently arrives at dynamic ensembles that reproduce the entire NMR dataset with exquisite accuracy and theoretically reveal the systems conformational space and equilibrium. The method as well as its potential towards configurational elucidation is presented on diastereomeric pairs of flexible molecules: a small 1,4-diketone 1 with a single rotatable bond as well as a 24-ring macrolide related to the natural product mandelalide A 2.

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44

Chen, Songhua, D. T. Georgi, Oscar Olima, Hector Gamin y J. C. Minetto. "Estimation of Hydrocarbon Viscosity With Multiple-te Dual-tw MRIL Logs". SPE Reservoir Evaluation & Engineering 3, n.º 06 (1 de diciembre de 2000): 498–508. http://dx.doi.org/10.2118/68021-pa.

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Summary We report a case study of using nuclear magnetic resonance (NMR) multiple-te, dual wait-time (tw) log acquisitions for quantitative characterization of San Jorge Basin reservoir oil viscosity. Previously, dual-tw logs have been used to discern gas and oil from water, while dual-te logs have been used as a qualitative light oil indicator. Although theoretically simple, quantitative determination of viscosity from dual-te logs is complicated by several factors, including poor signal-to-noise ratio, difficulties in separating oil from water, and the uncertainty of internal gradient strength. In the present study, multiple-te acquisitions of dual-tq logs were used to isolate the oil from the water signal. The values of viscosity of the reservoir fluids can be estimated from either intrinsic T2 or T1. In estimation of the apparent T2, we used a model that does not explicitly require knowledge of the internal gradient, thereby minimizing the effects arising from the uncertainty of the internal and tool gradient strengths. Because T1 and intrinsic T2 are estimated independently, the degree of agreement between the two values provides an indication of the reliability of the two estimates. The main example in the study of four pay zones was thought to contain viscous oil. However, our analysis indicated that the viscosity values of the oil are less than 5 cP. The predictions have been substantiated by production of light hydrocarbons from the three zones that have been perforated. Further, a good agreement is obtained for the viscosity estimates based on NMR log data and laboratory pressure/volume/temperature (PVT) analysis. Introduction Hydrocarbon viscosity is an important reservoir fluid parameter that significantly affects oil recovery and economics. Fluid flow is inversely proportional to viscosity and the higher the viscosity the lower the flow rate and the slower the recovery. Further, when two or more fluids are flowing, the ratio of the viscosities, the mobility ratio, is one of the key parameters that affects sweep efficiency and ultimate recovery. In many reservoirs, it is uneconomical to produce heavy, viscous oil, and thus it is crucial to determine oil viscosity before completing the well. The problem is even more pressing when oil viscosities vary within a hydrocarbon column or from zone to zone when attempting to commingle multiple zones. Many laboratory procedures can determine viscosity. Samples for viscosity determination may be obtained from reservoir fluid samples recovered from well tests or drill stem tests, downhole fluid samplers, or reconstituted from separator samples. Sampling procedures are generally limited to a few depths; and, of course, samples reconstituted from separators are associated with the entire producing interval and may not be associated with a single depth. Further, there is always the concern that the fluid samples may not be representative of the in-situ reservoir fluids. Nuclear magnetic resonance (NMR) logging measurements have the potential to provide in-situ viscosity measurement because the NMR relaxation times, T1 and T2, correlate strongly with fluid viscosity. The difficulty with NMR-determined viscosity is that the measurement is relatively shallow. Thus, the hydrocarbon saturations may be significantly reduced (So or Sor), and hence, the sought NMR oil signal is small. Further, the interpretation may be complicated by NMR signals originating from the invading fluids. NMR response is controlled by both rock and fluid properties. In fact, NMR log interpretation is complicated because it is not always clear whether the T2 decay reflects hydrocarbon and/or rock properties. However, we are fortunate because two NMR experiment parameters, te and tw (Fig. 1), can be used to tailor the NMR data acquisition to separate the hydrocarbon and rock property effects. One of the first specialized NMR applications that took advantage of the ability to control NMR log acquisition parameters was hydrocarbon typing.1 NMR hydrocarbon typing in porous media relies on the difference in NMR response in either or both of the relaxation times (T1 and T2) or diffusivity of oil, water, and gas. By carefully designing the logging program and using combinations of pulse sequences, one can enhance the relaxation and diffusivity contrasts between the different fluid phases. The two commonly used approaches for magnetic resonance image log (MRIL®)** based hydrocarbon typing are dual-tq logging1,2 and dual-te logging.3,4 Dual-tw logging utilizes the T1 contrast between nonwetting light hydrocarbons and the wetting water for quantitative light hydrocarbon typing, while dual-te logging utilizes the viscosity (and thus diffusivity) contrast between reservoir fluids. The latter, to our knowledge, previously has been used mainly as a qualitative, or, at best semiquantitative, hydrocarbon indicator. Quantitative estimation of oil viscosity and saturation require solving problems related to uncertainty of internal magnetic-field gradient and separating oil from water signals. We combined the dual-tw and dual-te approaches to maximize the advantages of both T1 and T2 contrasts. With multiple-te passes of dual-tw logs, we are able to eliminate a majority of the water signal from dual-tw logs; the remaining signal is predominantly an oil signal. The characterization of oil viscosity is achieved by analyzing the relaxation times and diffusion effect on the isolated oil signal with the multiple-te data acquisition. This reduces uncertainties due to the interfering water signal originating from either the irreducible and bound water or from the invading mud filtrate present on conventional dual-te logs.

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45

Do, Duy Minh, Wei Gao, Chongmin Song y Michael Beer. "Interval spectral stochastic finite element analysis of structures with aggregation of random field and bounded parameters". International Journal for Numerical Methods in Engineering 108, n.º 10 (1 de abril de 2016): 1198–229. http://dx.doi.org/10.1002/nme.5251.

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46

Adem, Ziad, Flavien Guenneau, Marie-Anne Springuel-Huet y Antoine Gédéon. "PFG NMR investigation of hydrocarbon diffusion in large NaX zeolite crystals: Effect of internal field gradients on diffusion data". Microporous and Mesoporous Materials 114, n.º 1-3 (septiembre de 2008): 337–42. http://dx.doi.org/10.1016/j.micromeso.2008.01.019.

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47

Mahour, Laxmi Narayan, M. Manjunatha, Harish Kumar Choudhary, Rajeev Kumar, A. V. Anupama, R. Damle, K. P. Ramesh y Balaram Sahoo. "Structural and magnetic properties of Al-doped yttrium iron garnet ceramics: 57Fe internal field NMR and Mössbauer spectroscopy study". Journal of Alloys and Compounds 773 (enero de 2019): 612–22. http://dx.doi.org/10.1016/j.jallcom.2018.09.213.

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48

Almenningen, Stian, Srikumar Roy, Arif Hussain, John Georg Seland y Geir Ersland. "Effect of Mineral Composition on Transverse Relaxation Time Distributions and MR Imaging of Tight Rocks from Offshore Ireland". Minerals 10, n.º 3 (3 de marzo de 2020): 232. http://dx.doi.org/10.3390/min10030232.

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In this paper, we investigate the effect of magnetic field strength on the transverse relaxation time constant (T2) in six distinct core plugs from four different rock types (three sandstones, one basalt, one volcanic tuff and one siltstone), retrieved from offshore Ireland. The CPMG pulse-sequence was used at two different magnetic field strengths: high-field at 4.70 T and low-field at 0.28 T. Axial images of the core plugs were also acquired with the RAREst sequence at high magnetic field strength. Thin-sections of the core plugs were prepared for optical imaging and SEM analysis, and provided qualitative information on the porosity and quantification of the elemental composition of the rock material. The content of iron varied from 4 wt. % to close to zero in the rock samples. Nevertheless, the effective T2 distributions obtained at low-field were used to successfully predict the porosity of the core plugs. Severe signal attenuations from internal magnetic gradients resulted in an underestimation of the porosity at high-field. No definitive trend was identified on the evolution of discrete relaxation time components between magnetic field strengths. The low-field measurements demonstrate that NMR is a powerful quantitative tool for petrophysical rock analysis as compared to thin-section analysis. The results of this study are of interest to the research community who characterizes natural gas hydrates in tight heterogeneous core plugs, and who typically relies on MR imaging to distinguish between solid hydrates and fluid phases. It further exemplifies the importance of selecting appropriate magnetic field strengths when employing NMR/MRI for porosity calculation in tight rock.

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49

Buffam, D. J., T. S. Sorensen y S. M. Whitworth. "A study of the observable axial- and equatorial-2,4-dimethyl-2-adamantyl cations. Further examples of equilibrating nonclassical structures in "ordinary" tertiary carbocations". Canadian Journal of Chemistry 68, n.º 10 (1 de octubre de 1990): 1889–93. http://dx.doi.org/10.1139/v90-292.

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The title cations have been prepared insitu from the corresponding tertiary alcohols, using three different superacid systems. The 13C NMR spectra were measured at a number of different temperatures, using the structurally related 13C peaks of internal 2-methyl-2-adamantyl cation as a reference. Using this protocol, one finds that the 13C chemical shifts of the four β carbons are quite temperature dependent, two of them shifting to higher field and the other two on the opposite face shifting to lower field, or vice versa, as the temperature is increased or decreased. This behavior is uniquely ascribed to a rapid equilibration between two "structures" for each cation system. The two structures in question involve selective C–C hyperconjugative delocalization on one face or the other of the 2-adamantyl skeleton. In the case of the 4-equatorial-methyl isomer, a structure with C–C hyperconjugation on the methyl-substituted face is most stable, while for the 4-axial-methyl cation, the structure with delocalization from the face opposite the substituent is favored. The former preference is regarded as stereoelectronic in origin while the latter is ascribed to steric effects. Keywords: non-planar carbocations, temperature dependent NMR shifts, stereoelectronic effects.

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50

Pranolo, Sunu Herwi, Muhammad Tasmiul Khoir y Muhammad Fahreza Pradhana. "Production of clean synthetic gas from palm shell in a fixed bed gasifier with recycle system of producer gas". MATEC Web of Conferences 197 (2018): 09004. http://dx.doi.org/10.1051/matecconf/201819709004.

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Tar as side product of biomass gasification could potentially degrade internal combustion engine performance if syngas is used for the fuel. Tar reduction may be achievable with recycling of outlet producer gas back into the gasifier. This research studied the effects of recycle system to tar content in syngas as product of palm shell gasification in a fixed bed gasifier. The effects of recycling system were examined using gasification of palm shell with primary air at 3.30 Nm3/h, mixture of primary air at 1.80 Nm3/h and secondary air at 1.50 Nm3/h, and mixture of primary air and recycled gas. Volumetric rate of recycle gas were varied at 0.90 and 1.20 Nm3/h respectively. Gasification performance evaluation was based on Specific Gasification Rate. Syngas quality was rated with tar content, CO, CH4, H2, CO2, and N2 composition. The highest Specific Gasification Rate of 111.71 kg/m2h and tar reduction up to 61.95% were achieved using recycle system at volumetric rate of 0.90 Nm3/h with temperature of operation is 750°C. The highest heating value of 6.34 MJ/Nm3 was attained using recycled gas volumetric rate at 1.20 Nm3/h.

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