Applied Magnetic Resonance最新文献

The matrix pencil method (MPM) is a powerful tool for processing transient nuclear magnetic resonance (NMR) relaxation signals with promising applications to increasingly complex problems. In the absence of signal noise, the eigenvalues recovered from an MPM treatment of transient relaxometry data reduce to relaxation coefficients that can be used to calculate relaxation time constants for known sampling time ∆t. The MPM eigenvalue and relaxation coefficient equality as well as the resolution of similar eigenvalues and thus relaxation coefficients degrade in the presence of signal noise. The relaxation coefficient ∆t dependence suggests one way to improve MPM resolution by choosing ∆t values such that the differences between all the relaxation coefficient values are maximized. This work develops mathematical machinery to estimate the best ∆t value for sampling damped, transient relaxation signals such that MPM data analysis recovers a maximum number of time constants and amplitudes given inherent signal noise. Analytical and numerical reduced dimension MPM is explained and used to compare computer-generated data with and without added noise as well as treat real measured signals. Finally, the understanding gleaned from this effort is used to predict the best data sampling time to use for non-discrete, distributions of relaxation variables.

Battery fast charging is pivotal for broader acceptance of electric mobility. While demonstrated for lithium titanate ((text {Li}_4text {Ti}_5text {O}_{12},) LTO) anodes, the underlying mechanisms are still poorly understood. Recently, NMR (T_1) relaxation time constants of ({}^7text {Li}) in the bulk of LTO were found to change if the surrounding electrolyte was altered. It was explained by interdiffusion of mobile lithium ions between the two phases, facilitated by unpinning of polarons from surface defects and leading to a pseudocapacitive effect that potentially influences fast charging. This effect is explored further by systematically varying the lithium salt concentration in an aprotic electrolyte in contact with LTO. Spectrally resolved ({}^7text {Li}) (T_1) NMR relaxation times were used as a measure for bulk concentration changes of paramagnetic polaronic charges in LTO. Correlation of electrolyte concentration and ({}^7text {Li}) (T_1) showed qualitatively different behavior above and below a salt concentration of about 5 mM, leading to a relaxation dispersion maximum in the LTO bulk. At intermediate concentrations, relaxation was consistent with a ({}^7text {Li}) exchange equilibrium between LTO and electrolyte. Upon contact of the two phases, yet without insertion into an electrochemical cell or applying an external potential, lithium ions redistributed between LTO bulk and liquid electrolyte. The results can be understood analogously to the distribution of mobile lithium ions between two phases separated by a (text {Li}^{+}) permeable membrane. This is the first demonstration of such an equilibrium for non-faradaic lithium exchange at an interface between a solid ceramic electrode and a liquid electrolyte outside an electrochemical cell, substantiating our previous hypothesis of a polaron-supported mechanism. This study provides a basis for more quantitative (surface)-defect engineering, which is key to optimize battery fast-charging properties.

A new variant of the superconducting fulleride Rb(_3)C(_{60}) is presented, with (^3)He atoms encapsulated in the C(_{60}) cages. The (^3)He nuclei act as sensitive NMR probes embedded in the material. The superconducting and normal states are characterized by (^3)He NMR. Evidence is found for coexisting vortex liquid and vortex solid phases below the superconducting transition temperature. A strong dependence of the spin–lattice relaxation time constant on spectral frequency is observed in the superconducting state, as revealed by two-dimensional NMR utilizing an inverse Laplace transform. Surprisingly, this phenomenon persists, in attenuated form, at temperatures well above the superconducting transition.

Measurement of the liquid-solid mass transfer coefficient within a trickle bed (i.e. gas-liquid flow within a packed bed) of porous silica pellets is achieved through the use of ({T}_{2}-{T}_{2}) relaxation exchange nuclear magnetic resonance (NMR). Compared to many conventional measurement techniques, the NMR method enables measurement of mass transport using pellets of real commercial interest. Mass transfer coefficients measured using the NMR technique over a range of liquid Reynolds number, 0.2 (le R{e}_{mathrm{L}}le ) 1.4, are compared to a number of literature correlations, with values measured using the NMR method falling within the range predicted by the correlations. The results demonstrate the importance of considering both the flow conditions and the type of pellets used to develop mass transport correlations in trickle beds. This novel NMR application may be utilized in the future to screen catalyst pellets in trickle beds for optimal mass transport properties.

Radio-frequency Amplification by Stimulated Emission of Radiation (RASER) is a promising tool to study nonlinear phenomena or measure NMR parameters with unprecedented precision. Magnetic fields, J-couplings, and chemical shifts can be recorded over long periods of time without the need for radiofrequency excitation and signal averaging. One key feature of RASER NMR spectroscopy is the improvement in precision, which grows with the measurement time (T_{{text{m}}}^{3/2}), unlike conventional NMR spectroscopy, where the precision increases with (T_{{text{m}}}^{1/2}). However, when detecting NMR signals over minutes to hours, using available NMR magnets (ppb homogeneity), the achieved frequency resolution will eventually be limited by magnetic field fluctuations. Here, we demonstrate that full compensation is possible even for open low-field electromagnets, where magnetic field fluctuations are intrinsically present (in the ppm regime). A prerequisite for compensation is that the spectrum contains at least one isolated RASER line to be used as a reference, and the sample experiences exclusively common magnetic field fluctuations, that is, ones that are equal over the entire sample volume. We discuss the current limits of precision for RASER NMR measurements for two different cases: The single-compartment RASER involving J-coupled modes, and the two-compartment RASER involving chemically shifted species. In the first case, the limit of measurable difference approaches the Cramér-Rao lower bound (CRLB), achieving a measurement precision ({sigma }_{f}<{10}^{-4}) Hz. In the second case, the measured chemical shift separation is plagued by independently fluctuating distant dipolar fields (DDF). The measured independent field fluctuation between the two chambers is in the order of tens of mHz. In both cases, new limits of precision are achieved, which paves the way for sub-mHz detection of NMR parameters, rotational rates, and non-linear phenomena such as chaos and synchrony.

The hydrodynamics in packed reactors strongly influences reactor performance. However, limited experimental techniques are capable of non-invasively measuring the velocity field in optically opaque packed beds at the turbulent flow conditions of commercial relevance. Here, compressed sensing magnetic resonance velocity imaging has been applied to investigate the hydrodynamics of turbulent flow through narrow packed beds of hollow cylindrical catalyst support pellets as a function of the tube-to-pellet diameter ratio, (N), for (N=) 2.3, 3.7, and 4.8. 3D images of time-averaged velocity for the gas flow through the beds were acquired at constant Reynolds number, (R{e}_{mathrm{p}}=) 2500, at a spatial resolution of 0.70 mm ((tt x)) (times) 0.70 mm ((tt y)) (times) 1.0 mm ((tt z)). The resulting flow images give insight into the bed and pellet scale hydrodynamics, which were systematically compared as a function of (N). Some changes in hydrodynamics with (N) were observed. Namely, the near-wall hydrodynamics changed with (N), with the (N=) 4.8 bed showing higher velocity at the wall compared to the (N=) 2.3 and (N=) 3.7 beds. Further, in the (N=) 3.7 bed, channels of high velocity, termed flow lanes, were found 1.3 particle diameters from the wall, possibly due to the bed structure in this particular bed. At the pellet scale, the hydrodynamics were found to be independent of (N). The results reported here demonstrate the capability of magnetic resonance velocity imaging for studying turbulent flows in packed beds, and they provide fundamental insight into the effect of (N) on the hydrodynamics.

Applications of time-domain ¹H NMR spin-diffusion experiments for studying morphology of industrially relevant polymers are reviewed. The method exploits the contrast in molecular mobility in different phases in multi-phase organic materials, which could be in some cases advantageous to traditional morphological methods. A brief overview of different time-domain spin-diffusion methods and data analysis is provided. The effect of domain size distributions and their clustering, which were previously analyzed by numerical simulations of spin-diffusion curves, is discussed. Examples of different types of morphology in polymers with hard and soft domains are presented, namely, lamellar morphology and its changes during annealing; interfacial layers in different types of polymers; fragmented structure of crystal lamellae in isotactic polybutene-1 and its copolymer with form I crystals; fibrillar morphology of melt-spun Nylon 6 and poly(ethylene terephthalate) fibers; morphology of gel-spun ultra-high-molecular-weight polyethylene fibers; ionic clusters in polymeric ionomers; the rubber–filler interface in filled rubbers; the structure of network of physical junctions in filled rubbers and ionomers; and morphology of thermoplastic polyurethanes. Domain sizes from the NMR method are compared with those determined for the same materials by small-angle X-ray scattering and transmission electron microscopy. All results are in good agreement. In addition to domain sizes, the NMR method provides several details of polymer morphology, namely, morphological heterogeneities, the type and the thickness of interfacial layers, the presence of (sub)nano-domains, and molecular mobility in different phases. Thus, the method offers information that is complementary to the conventional methods. The effect of structural heterogeneities on macroscopic properties is briefly discussed.