Large magnetocaloric effect and magnetic properties of polymorphic RCrO4 (R=rare earth) oxides

Trabajo presentado al: "International Symposium on the Reactivity of Solids" celebrado en San Petersburgo (Rusia) del 9 al 13 de junio de 2014.-- et al.RCrO4 oxides, where R=rare earth, crystallize at ambient conditions with the tetragonal zircon-type structure, S.G. I41/amd. Recently, we have succeeded to synthesize the scheelite polymorphs (S.G. I41/a) for most of the R elements, by treating the zircon phases at 40 kbar and 813 K. These scheelite high pressure forms are quencheable after releasing the pressure, and TGA and X-ray diffraction data reveal a reversible transition from scheelite polymorph to zircon at 700 K. The coexistence of two paramagnetic ions in these phases, namely Cr5+ and R3+, constitutes a very interesting scenario to study 3d-4f magnetic interactions. In this sense, most of the zircon-type RCrO4 oxides behave as ferromagnetic, while the scheelite polymorphs are antiferromagnetic. The change in the sign of the magnetic interaction can be explained by considering the changes in both distances and bond angles of Cr-O-R pathway through which the superexchange interactions take place. Neutron diffraction studies have been used to determine the nuclear and magnetic structures for these zircon and scheelite RCrO4 polymorphs. The analysis of the data reveals the onset of new reflections below the estimated Néel temperature from the previous magnetic susceptiblity data corresponding to the scheelite phases. The magnetic structure for these scheelite RCrO4 oxides can be described with a propagation vector κ = [0 0 0], where the moments of R3+ and Cr5+ are aligned along the c-axis or confined in the ab-plane of the tetragonal structure depending on the nature of the R element. This behavior has been confirmed from heat capacity measurements where the onsets of λ-anomalies at the ordering temperatures are almost coincident with those determined from magnetic susceptibility and neutron diffraction analysis. A detailed analysis of the zircon and scheelite structural types is included in this work to explain the different magnetic behavior showed by these two polymorphic phases allowing establishing relationships structure-magnetic properties. Very recently it has been reported that the zircon RCrO4 (R= Ho and Dy) phases show large values of the magnetocaloric parameters that make of these oxides potential refrigerant materials to be used for the liquefaction of hydrogen. It is noting that the field induced magnetization in the case of the scheelite polymorphs yields magnetic moments larger than in the case of the homologous zircon phases and hence the magnetocaloric effect is expected to be larger for the scheelite polymorphs in comparison with the zircon ones. In this work a preliminary study has been done in order to determine the magnetocaloric parameters of the two polymorphic phases of TbCrO4 and HoCrO4. In this sense the isothermal entropy change, ΔST, as a function of temperature has been calculated for magnetic field variations from the measurements of magnetization versus field at different temperatures. Both magnetocaloric parameters, ΔST and the adiabatic temperature change, ΔTad, have also been evaluated from the entropy functions at different fields obtained from the heat capacity data.Peer Reviewe

A comprehensive study of a versatile magnetic refrigeration demonstrator

A versatile room temperature reciprocating magnetic refrigeration demonstrator has been designed, built and tested in order to evaluate the influence of different running parameters and to check suitable magnetocaloric materials for cooling at room temperature. A comprehensive study has been done with Gd spheres of 0.2-0.4 mm diameter arranged as a double regenerator with 15 g each. A Halbach Nd2e14B permanent magnet with a slot of 10 mm width has been used to generate the magnetic field with a maximum value of 1.4 T. The heat transfer fluid is a mixture of water and ethylene glycol in a 75-25 percentage. The demonstrator achieves a maximum no-load temperature span close to 20 K, with a regeneration ratio of ¿ = 4.1, and a maximum cooling power Qc=6 W at zero temperature span. COP values have been shown and different thermodynamic AMR cycles have been studied looking for the best parameters

Application of simulations to thermodynamic properties of materials for magnetic refrigeration: A calorimetric approach to material’s magnetocaloric parameters

A magnetic refrigeration system is a complex system that involves the magnetocaloric effect (MCE) and the heat transfer problems working in a coupled manner. For this purpose, characterization of materials showing MCE is needed. Calorimetric characterization allows the obtention of thermodynamic variables needed for a precise quantification of this effect. More specifically, in systems with continuous magnetic field variation, in order to calculate the heat generation due to MCE, the knowledge of their magnetocaloric parameters—adiabatic temperature change (Formula presented.) and isothermal entropy change (Formula presented.)—and the heat capacity (Formula presented.), for every temperature and magnetic field present is needed. In this work, (Formula presented.) family materials have been either characterized or interpolated and used in numerical simulations in COMSOL Multiphysics™ software. The characterization was carried out with measurements of (Formula presented.), (Formula presented.) and (Formula presented.) and the calculation of other derived parameters, at different temperatures and magnetic fields

Heatmaps in soccer: event vs tracking datasets

We investigate how similar heatmaps of soccer players are when constructed from (i) event datasets and (ii) tracking datasets. When using event datasets, we show that the scale at which the events are grouped strongly influences the correlation with the tracking heatmaps. Furthermore, there is an optimal scale at which the correlation between event and tracking heatmaps is the highest. However, even at the optimal scale, correlations between both approaches are moderate. Furthermore, there is high heterogeneity in the players' correlation, ranging from negative values to correlations close to the unity. We show that the number of events performed by a player does not crucially determine the level of correlation between both heatmaps. Finally, we analyzed the influence of the player position, showing that defenders are the players with the highest correlations while forwards have the lowest.Comment: 6 pages, 5 figure

Diagnóstico de Leishmaniosis canina mediante la técnica de reacción en cadena de la polimerasa (PCR) : un procedimiento simple para uso en la clínica

Para detectar la presencia de Leishmania ssp, en distintas muestras clínicas de perros sospechosos de padecer esta enfermedad, se ha utilizado la técnica PCR, amplificando para ello un fragmento del gen SSU rRNA, repetido más de 100 veces en el genoma del parásito. El método se ha optimizado para utilizarlo como método de rutina en la clínica. El procesado de las muestras es rápido y simple. El diagnóstico se ha realizado por presencia/ausencia del parásito, utilizando para ello muestras de sangre, médula ósea y ganglio linfático principalmente. En el caso de existir el parásito en el huésped se visualiza una banda nítida de un tamaño de 603 bp y en el caso de que el parásito esté ausente, no se detecta la presencia de esta banda. Los mejores resultados se obtuvieron cuando la muestra de partida fue médula o ganglio linfático. El método presenta la ventaja adicional de detectar portadores asintomáticos, incluidos los titulas de IFI dudosos. La técnica PCR se presenta como test de diagnóstico rutinario, siendo más rápida, eficaz y económica que los métodos de diagnóstico clásicos

Empty Urbanism: the bursting of the Spanish housing bubble

The depth of the Spanish housing crisis manifests itself in the collapse of construction activity and in the amount of housing and land stocks. The geography of the crisis shows its widespread nature, and the intensity of the previous bubble explains spatial differences. Resulting from this collapse are some problematic areas of 'empty urbanism'. An enormous land bubble, emerging from the peculiar Spanish urban development model, was a key factor in the impacts - caused by the crisis - on the territory and land-use plans. The crisis has demonstrated the unsustainability of this and the urgency of change in the existing land-use plans

Presence of Clostridium difficile in pig faecal samples and wild animal species associated with pig farms

Aims: to determine the presence of Clostridium difficile on fattening pig farms in north-eastern Spain. Methods and results: Twenty-seven farms were sampled. Pools of pig faecal samples (n = 210), samples of intestinal content from common farm pest species (n = 95) and environment-related samples (n = 93) were collected. Isolates were tested for toxin genes of C. difficile, and typed by PCR-ribotyping and toxinotyping. The minimal inhibitory concentrations of six antimicrobial agents were determined using Etest. Thirty-four isolates were obtained from 12 farms, and 30 (88·2%) had toxin genes. Seven ribotypes were identified. Ribotype 078 and its variant 126 were predominant (52·9%). The same ribotypes were isolated from different animal species on the same farm. None of the isolates were resistant to metronidazole or vancomycin. Conclusions: Clostridium difficile was common within the pig farm environment. Most of the positive samples came from pest species or were pest-related environmental samples. Significance and Impact of the Study: Pest species were colonized with toxigenic and antimicrobial-resistant C. difficile strains of the same ribotypes that are found in humans and pigs. Rodents and pigeons may transmit toxigenic and antimicrobial-resistant C. difficile strains that are of the same ribotypes as those occuring in humans