Search

Modifications and updates to an existing X-band dual-polarization phased-array weather radar. Efforts to improve the polarization isolation of the antenna. Characterization via near-field range and field measurements. Dual-polarized phased-array radars used for weather retrieval purposes is an emerging tendency over the last few years. The dual-polarization technology provides an expanded range of weather products and the reliability of these polarimetric products rely on the beam shape quality of the system under use and its polarization isolation. This thesis presents the calibration process of a one-dimensional scanning phased-array radar to assure its beamforming quality. A cross-polarization cancellation technique, with no additional hardware requirements, is tested and appeared to optimize the array settings for an improved isolation and therefore, for a better data quality. Radares formados por agrupaciones de antenas que trabajan con dos polarizaciones con la intención de tomar medidas meteorológicas es una tendencia a la alza en los últimos años. El hecho de utilizar dos polarizaciones ofrece una gama ampliada de productos meteorológicos y la fiabilidad de estos dependerá de la cualidad del haz radiado por el radar y del aislamiento entre las polarizaciones de éstos. Esta tesis presenta el proceso de calibración de un radar, formado por una agrupación de antenas que escanean en una dimension, con tal de garantir la calidad de los haces producidos. Una técnica para cancelar la polarización no deseada, sin la necesidad de ningún hardware adicional, ha sido probada y ha mostrado que puede mejorar la configuración de la agrupación para obtener una mejora del aislamiento y mejorar así, la cualidad de los datos. Radars formats per agrupacions d’antenes que treballen amb dues polarizations amb l’intenció de mesurar paràmetres meteorològics és una tendencia que va en augment en els últims anys. El fet d’utilitzar dues polarizations ofereix una gama ampliada de productes meteorològics i la fiabilitat d’aquests dependrà de la qualitat que tinguin els feixos radiats pel radar així com de l’aïllament entre les polaritzacions d’aquests. Aquesta tesis presenta el procés de calibració d’un radar, format per una agrupació d’antenes que escanejen en una dimensió, per tal de garantir la qualitat dels feixos produïts. Una tècnica per cancelar la polarització no desitjada, sense la necessitat de hardware adicional, ha estat provada i ha mostrat que pot millorar la configuració de l’agrupació per tal d’obtenir una millora en el l’aïllament i així millorar-ne la qualitat de les dades.

It has recently been shown how dynamic nuclear polarization can be used to hyperpolarize the bulk of proton-free solids. This is achieved by generating the polarization in a wetting phase, transferring it to nuclei near the surface and relaying it towards the bulk through homonuclear spin diffusion between weakly magnetic nuclei. Pulse cooling is a strategy to achieve this that uses a multiple contact crosspolarization sequence for bulk hyperpolarization. Here, we show how to maximize sensitivity using the pulse cooling method by experimentally optimizing pulse parameters and delays on a sample of powdered SnO2. To maximize sensitivity we introduce an approach where the magic angle spinning rate is modulated during the experiment: the CP contacts are carried out at a slow spin rate to benefit from faster spin diffusion, and the spin rate is then accelerated before detection to improve line narrowing. This method can improve the sensitivity of pulse cooling for Sn-119 spectra of SnO2 by an additional factor of 3.5. (C) 2019 Elsevier Inc. All rights reserved.

The authors want to thank Dr. Jean-Nicolas Dumez for helpful discussions; Sven Sieber, Michael Schenkel, Rolf Hensel, Jacco van Beek, Ion Prisucaru, and Armin Purea for contributions to the system design and implementation; and Frank Engelke, Pietro Lendi, Klemens Kessler, Daniel Eckert, Daniel Guy Baumann, Dirk Wilhelm, Roberto Seydoux, Uwe Wagner, Cengiz Cetrefli, Tonio Gianotti, and Jorg Hinderer for contributions to the system specification. Some of the work in this article employed the SpinDynamica code for Mathematica, programmed by Malcolm H. Levitt, with contributions from Jyrki Rantaharju, Andreas Brinkmann, and Soumya Singha Roy, available at www.SpinDynamica.soton.ac.uk.; International audience; We present novel means to hyperpolarize deuterium nuclei in 13CD2 groups at cryogenic temperatures. The method is based on cross-polarization from 1H to 13C and does not require any radio-frequency fields applied to the deuterium nuclei. After rapid dissolution, a new class of long-lived spin states can be detected indirectly by 13C NMR in solution. These long-lived states result from a sextet-triplet imbalance (STI) that involves the two equivalent deuterons with spin I = 1. An STI has similar properties as a triplet-singlet imbalance that can occur in systems with two equivalent I = 12/ spins. Although the lifetimes T STI are shorter than T 1(Cz), they can exceed the life-time T 1(Dz) of deuterium Zeeman magnetization by a factor of more than 20.

International audience; Competitive adsorption of dilute quantities of certain organic molecules and water at silicate surfaces strongly influence the rates of silicate dissolution, hydration, and crystallization. Here, we determine the molecular-level structures, compositions, and site-specific interactions of adsorbed organic molecules at low absolute bulk concentrations on heterogeneous silicate particle surfaces at early stages of hydration. Specifically, dilute quantities (similar to 0.1% by weight of solids) of the disaccharide sucrose or industrially important phosphonic acid species slow dramatically the hydration of low-surface-area (similar to 1 m(2)/g) silicate particles. Here, the physicochemically distinct adsorption interactions of these organic species are established by using dynamic nuclear polarization (DNP) surface-enhanced solid-state NMR techniques. These measurements provide significantly improved signal sensitivity for near-surface species that is crucial for the detection and analysis of dilute adsorbed organic molecules and silicate species on low-surface-area particles, which until now have been infeasible to characterize. DNP-enhanced 2D Si-29{H-1}, C-13{H-1}, and P-31{H-1} heteronuclear correlation and 1D Si-29{C-13} rotational-echo double-resonance NMR measurements establish hydrogen-bond-mediated adsorption of sucrose at distinct nonhydrated and hydrated silicate surface sites and electrostatic interactions with surface Ca2+ cations. By comparison, phosphonic acid molecules are found to adsorb electrostatically at or near cationic calcium surface sites to form Ca(2+)phosphonate complexes. Although dilute quantities of both types of organic molecules effectively inhibit hydration, they do so by adsorbing in distinct ways that depend on their specific architectures and physicochemical interactions. The results demonstrate the feasibility of using DNP-enhanced NMR techniques to measure and assess dilute adsorbed molecules and their molecular interactions on low-surface-area materials, notably for compositions that are industrially relevant.