Color-center–hosting semiconductors are emerging as promising source materials for low-field dynamic nuclear polarization (DNP) at or near room temperature, but hyperfine broadening, susceptibility ...to magnetic field heterogeneity, and nuclear spin relaxation induced by other paramagnetic defects set practical constraints difficult to circumvent. Here, we explore an alternate route to color-center–assisted DNP using nitrogen-vacancy (NV) centers in diamond coupled to substitutional nitrogen impurities, the so-called P1 centers. Working near the level anticrossing condition—where the P1 Zeeman splitting matches one of the NV spin transitions—we demonstrate efficient microwave-free 13C DNP through the use of consecutive magnetic field sweeps and continuous optical excitation. The amplitude and sign of the polarization can be controlled by adjusting the low-to-high and high-to-low magnetic field sweep rates in each cycle so that one is much faster than the other. By comparing the 13C DNP response for different crystal orientations, we show that the process is robust to magnetic field/NV misalignment, a feature that makes the present technique suitable to diamond powders and settings where the field is heterogeneous. Applications to shallow NVs could capitalize on the greater physical proximity between surface paramagnetic defects and outer nuclei to efficiently polarize target samples in contact with the diamond crystal.
Alkanes, due to their hydrophobicity, are extensively used in NMR experiments as probe molecules for geometric tortuosity measurement of polar mesopores. In this work, ab-initio calculations results ...show that the interactions between linear and cyclic alkanes with the walls of silica mesopores are non-negligible. The dependence of the adsorption energy with the molecular length and shape is determined. Furthermore, a geometrical model is used to calculate a weighted adsorption energy that takes into account the fraction of molecules that reside close to the pore’s wall compared to those in the pore’s bulk. A dependence of the weighted energy with the pore diameter is found to correlate with restricted diffusion coefficients determined previously by nuclear magnetic resonance experiments. We conclude that the determination of the system’s geometric tortuosity by this methodology is not accurate for pore diameters smaller than ca. 6 nm.
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•Calculations show that alkanes interact with silica surfaces in a non-negligible way.•Alkanes closely attached to the pores’ walls diffuse slower than the bulk liquid.•A geometrical model reveals that for thin pores those interactions become decisive.•The reduced mobility of a portion of the alkanes blur NMR measurements of diffusivity.•We propose a diameter limit for the use of alkanes to measure geometrical tortuosity.
The dynamics and spatial distribution of water molecules in Nafion 117 membrane has been investigated by 1H NMR. To this end, 1D and 2D spin-spin (T2) relaxometry experiments were carried out as a ...function of the relative humidity ranging from 9 to 100%. The inverse Laplace transform was successfully applied to obtain the 1D T2 distributions and 2D relaxation maps. The 1D T2 distributions show two peaks at low RH and mainly one at high water content, which can be associated with the rearrangement of the exchange sites inside the polymeric channels with the amount of water. From the T2 distribution at 70% RH three types of water were identified corresponding to different degree of molecular mobility. The 2D T2-T2 maps show molecular exchange between the two water populations found at low RH, while at 70% the exchange between only the two more mobile water populations takes place.
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•The dynamics and spatial distribution of water molecules in Nafion 117 membrane has been investigated by NMR.•Spin-spin relaxometry experiments were carried out for different moisture conditions.•The inverse Laplace transform was successfully applied to obtain the 1D T2 distributions and 2D relaxation maps.•Three types of water were identified according to the interaction with the sulfonic groups.
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•T2-T2 NMR experiments are useful to study molecular exchange in porous media.•2D NMR experiments were successfully applied to organic materials.•Two algorithms for data processing by ...Numerical Laplace inversion methods are tested under several conditions.•Analytical results are confronted to numerical and experimental results.
Two-dimension (2D) Nuclear Magnetic Resonance relaxometry experiments are a powerful tool extensively used to probe the interaction among different pore structures, mostly in inorganic systems. The analysis of the collected experimental data generally consists of a 2D numerical inversion of time-domain data where T2-T2 maps are generated. Through the years, different algorithms for the numerical inversion have been proposed. In this paper, two different algorithms for numerical inversion are tested and compared under different conditions of exchange dynamics; the method based on Butler–Reeds–Dawson (BRD) algorithm and the fast-iterative shrinkage-thresholding algorithm (FISTA) method. By constructing a theoretical model, the algorithms were tested for a two- and three-site porous media, varying the exchange rates parameters, the pore sizes and the signal to noise ratio. In order to test the methods under realistic experimental conditions, a challenging organic system was chosen. The molecular exchange rates of water confined in hierarchical porous polymeric networks were obtained, for a two- and three-site porous media. Data processed with the BRD method was found to be accurate only under certain conditions of the exchange parameters, while data processed with the FISTA method is precise for all the studied parameters, except when SNR conditions are extreme.
The influence of molecular exchange in porous polymeric systems on experiments that simultaneously correlate and determine pore length scales and surface relaxivities is studied. Pore length scales ...are determined by taking advantage of the internal field gradients generated by the difference in magnetic susceptibility between the polymer matrix and the contained water, namely the Decay Due to the Internal Field experiment. Transverse relaxation is codified in the detection period thus enabling correlation with the pore length. For the hierarchical very well connected porous polymer system, the considerable molecular exchange of water between different environments renders signals in the two-dimensional maps that make the determination of the surface relaxivity a complex problem.
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•Two dimensional pore size/relaxation experiments provide surface relaxivity.•Porous polymer systems present a hierarchical well connected pore size distribution.•Water exchange between different pores appears as cross-peaks in d-T2 maps.
•Rocks and kerogens from Vaca Muerta formation are studied by NMR and XPS.•Analysis is focused on the speciation of carbon, oxygen, nitrogen and sulphur.•Differences of aromaticity obtained by both ...techniques are related to C-O content.•Results are compared with programmed pyrolysis and solid bitumen reflectance.•Changes of nitrogen and sulphur content with maturity are discussed.
The Vaca Muerta Formation, located in the Neuquén.
Basin (Argentina), is one of the most important shale plays in the world. A detailed structural characterization of the formation and evolution of the organic matter during maturation is a must for the correct evaluation of the petroleum potential. The present work studied the molecular characteristics of the organic matter of samples from several wells within the formation. Speciation of organic carbon, oxygen, nitrogen and sulfur present in the rocks was analyzed by XPS and 13C solid-state NMR spectroscopy and compared to data from more typical geochemical analyses, including pyrolysis and solid bitumen reflectance. Samples present with high TOC and cover a wide range of maturity levels (%BRo between 0.7 and 1.6%). Differences in carbon aromaticity measured by 13C NMR and XPS are discussed. Carbon, nitrogen, and sulfur present in organic isolates were studied and compared to reported data for several other shale formations (Duvernay, Bakken, Woodford and from the Bohai Bay Basin). The results are a novel contribution to the understanding and modeling of organic matter in the Vaca Muerta Formation and can be used to support and improve the classification of kerogens based on their molecular structures.
One of the key parameters in assessing the production potential of unconventional oil and gas reservoirs is the quantification of organic matter, which is generally determined by pyrolysis. ...Laboratory nuclear magnetic resonance (NMR) is widely used in the oil industry; being a nondestructive method, it enables the possibility to perform several analyses on the same sample, which is desirable due to the heterogeneity of these types of geological formations. 13C spectra provide much information, from speciation to maturity determination. The main drawback of this technique is the inherently low signal obtained for these nuclei, rendering extremely long experimental times. A solution is using cross-polarization schemes that transfer the high polarization of nearby 1H nuclei to 13C ones, with the penalty that quantification is a priori not feasible. In this work, we analyze the cross-polarization evolution and conclude that quantifying the total organic content (TOC) is possible for a wide range of organic matter content and maturities for rock samples corresponding to the Neuquén and Cuyo basins in Argentina.
Gestao de marcas: um estudo de caso da puma Ferraresi, Ana Lúcia; Franzoni, Ana Maria B; Amin Helou Filho, Esperidiao ...
Anales de la Universidad Metropolitana,
07/2011, Letnik:
11, Številka:
2
Journal Article
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