Práctica materiales ferroeléctricos relaxores

El objetivo de la práctica es familiarizar a los estudiantes con las técnicas experimentales empleadas en la caracterización de materiales ferroeléctricos, y en concreto de los denominados ferroeléctricos relaxores. Se utilizará la espectroscopia de impedancia para determinar la evolución de la constante dieléctrica con la frecuencia y la temperatura, junto con medidas de la polarización eléctrica. El material objeto de estudio será un compuesto cerámico con estructura de tipo perovskita con comportamiento relaxor, precedido por un estudio inicial de un ferroeléctrico convencional

The electrocaloric effect of lead-free Ba1−y_{1-y}Cay_yTi1−x_{1-x}Hfx_xO3_3 from direct and indirect measurements

We report on the dielectric and electrocaloric properties of Ba$_{1-y}$Ca$_y$Ti$_{1-x}$Hf$_x$O$_3$ for compositions $0.12<x<0.18$ and $y=0.06$, as well as $x=0.15$ and $0<y<0.15$, synthesized by the conventional solid-state reaction method. The addition of Hf/Ca broadens the ferroelectric-paraelectric phase transition, while moving it toward room temperature. Two interferroelectric transitions are seen to converge, together with the ferroelectric-paraelectric phase transition, at ca. 335 K for $0.12<x_c<0.135$ and $y=0.06$. Consistently with the dielectric properties, the electrocaloric effect maximizes closer to room temperature with increasing Hf/Ca substitutions, which promote larger temperature spans. The electrocaloric responsivity gradually decreases from 0.2 to 0.1 K$~$mm$~$kV$^{-1}$ with the addition of Hf/Ca. A homemade quasi-adiabatic calorimeter is employed to measure "directly" the electrocaloric data, which are also calculated from polarization-versus-electric-field cycles using "indirect" standard procedures. The comparison between measured and calculated values highlights the importance of having access to direct methods for a reliable determination of the electrocaloric effect.Comment: 30 pages, 17 figure

Determination of the charge-ordered phases in LuFe2O4

High-resolution synchrotron powder diffraction and Fe K-edge x-ray resonant scattering (RXS) have been combined to determine both the magnitude and sequence of the Fe charge segregation in LuFe2O4. Two phases with different charge disproportionation were found below the so-called charge ordering transition temperature T CO 320 K. The crystal structure between 320 K and 200 K shows a symmetry where four non-equivalent iron sites with different valences were determined (Fe2.77+,Fe2.63+, Fe2.36+ and Fe2.26+ ). Below 200 K the structure further changes to the symmetry and six crystallographic sites for the iron atoms were found. The simplest valence distribution corresponds to a tri-modal one, where the six Fe atoms are grouped in couples with three valences Fe2.8+, Fe2.5+ and Fe2.2+. Both polar and anti-polar charge sequences have been discarded. © EPLA, 2014.Financial support from the Spanish MINECO (project No. MAT2012-38213-C02-01) and DGA (CAMRADS) is acknowledged.Peer Reviewe

Caracterización de óxidos de Aurivillius para refrigeración electrocalórica.

La necesidad de encontrar tecnologías de refrigeración alternativas, más eficientes y sostenibles que la actual basada en compresión de vapor, ha puesto en el foco la investigación de materiales de estado sólido con efectos calóricos. En concreto, los materiales electrocalóricos muestran cambios térmicos reversibles en función del campo eléctrico conocidos como efecto electrocalórico (EEC) y en la actualidad son muy prometedores para aplicaciones de refrigeración en las proximidades de temperatura ambiente (ejemplo: refrigerador doméstico). La actividad investigadora en estos materiales se ha centrado fundamentalmente en polímeros y/o óxidos con estructura de tipo perovskita basados en plomo, siendo las magnitudes del EEC reportadas hasta la fecha todavía no lo suficientemente grandes para ser aplicables en tecnologías de refrigeración. Además, existe un problema importante en relación a los métodos experimentales empleados para cuantificar el EEC. Aunque existen métodos de medida directos e indirectos, son éstos últimos los que se usan de forma mayoritaria por ser más rápidos a pesar de ser menos fiables. Este trabajo pretende contribuir al campo de investigación de los materiales electrocalóricos examinando nuevos óxidos ferroeléctricos basados en fases de Aurivillius (libres de plomo) mediante medidas de sus propiedades eléctricas y con métodos de medida directos e indirectos del EEC.<br /

From gravity cores to overpressure history: the importance of measured sediment physical properties in hydrogeological models

The development of overpressure in continental margins is typically evaluated with hydrogeological models. Such approaches are used to both identify fluid flow patterns and to evaluate the development of high pore pressures within layers with particular physical properties that may promote slope instability. In some instances, these models are defined with sediment properties based on facies characterization and proxy values of porosity, permeability or compressibility are derived from the existing literature as direct measurements are rarely available. This study uses finite-element models to quantify the differences in computed overpressure generated by fine-grained hemipelagic sediments from Gulf of Cadiz, offshore Martinique and Gulf of Mexico, and their consequences in terms of submarine slope stability. By comparing our simulation results with in-situ pore pressure data measured in the Gulf of Mexico, we demonstrated that physical properties measured on volcanic-influenced hemipelagic sediments underestimate the computed stability of a submarine slope. Physical properties measured on sediments from the study area are key to improving the reliability and accuracy of overpressure models, and when that information is not available literature data from samples with similar lithologies, composition and depositional settings enable better assessment of the overpressure role as a pre-conditioning factor in submarine landslide initiation

Characterization of competing distortions in YFe2O4

We report the structural changes of three YFe2O4-δ (δ<0.1) specimens using high resolution synchrotron x-ray powder diffraction between 80 and 300 K. All samples adopt a rhombohedral cell at room temperature (space group R3m). This cell becomes unstable for the three samples on cooling, and the oxygen-poor specimen (δ∼0.1) shows a single transition at 240 K. The nearly stoichiometric (δ≤0.03) compounds exhibit two structural transitions with decreasing temperature at about 240 and 200 K. Each transition is revealed by an anomaly in the heat capacity measurements and a jump in the electric resistivity. Below 240 K, a strong splitting of some diffraction peaks is accompanied by the occurrence of superstructure peaks that follow the propagation vector k=(1/7,-2/7,9/7). The cell symmetry is then triclinic, and the structural transition is characterized by an expansion of the c axis coupled to a contraction of the other two lattice parameters. There are 49 nonequivalent sites for Fe atoms with a maximum charge disproportionation of ∼0.5e-. Upon cooling at 200 K, the previous superstructure peaks begin to vanish, and finally they are replaced by a new set of superstructure peaks following the propagation vector k=(1/4,1/2,1/4) with respect to the rhombohedral cell. The transition is also reflected in sudden changes in the lattice parameters that seem to smooth the changes observed in the previous transition. The new cell is also triclinic, and there are 48 nonequivalent Fe sites with a maximum charge disproportionation of ∼0.7e-. Both phases coexist in a wide temperature range because this second transition is not completed at 80 K. A symmetry mode analysis indicates a complicated pattern for the charge distribution in the Fe sublattice of both distorted structures but clearly discard any bimodal distribution of only two types of Fe cations. Therefore, the sharp jumps in the electric resistivity at the phase transitions are clearly correlated with two different structural changes. Finally, the oxygen stoichiometry seems to be a key factor in the stabilization of the different distorted structures.We are thankful for financial support from the Spanish Ministerio de Economía y Competitividad (MINECO) (Projects No. MAT2012-38213-C02-1-2 and No. MAT2015-68760-C2-1-2, cofunded by the European Regional Development Fund [ERDF] allocated by the European Union) and Diputacion General de Aragón (DGA, project E69).Peer Reviewe

Subseafloor stratigraphic profiling and soil classification from piezocone tests: A case study in the Gulf of Lion (NW Mediterranean Sea)

We show the results provided by piezocone tests in determining the stratigraphic profile and the soil classification of two drilling sites in the outer shelf and the upper slope of the Gulf of Lion, PRGL2 and PRGL1, respectively. Correlations with grain-size data indicate that sleeve friction can be used for profiling fine-grained sediments (site PRGL1), whereas cone tip resistance is the most adequate for sequences made of alternations of coarse- and fine-grained intervals (site PRGL2). Normalized cone resistance and friction ratio proved to be also appropriate for soil stratigraphy as it depicts trends in the coarse fraction of the tested soil. Silts and clays present in similar proportions at site PRGL1 responded to piezocone testing as pure clays usually do. Consequently, classical soil classification methods resulted in erroneous interpretation of these sediments as clays, whereas classification of the heterogeneous deposits at PRGL2 was consistent with the grain size. When tied to a high-resolution seismic reflection profile, the stratigraphy interpreted from the piezocone profile matches with the main seismic sequences and discontinuities defined from seismic stratigraphy analysis. Graded bedding also matches with cone tip resistance and sleeve friction data

Composition, geometry, and emplacement dynamics of a large volcanic island landslide offshore Martinique: From volcano flank-collapse to seafloor sediment failure

Landslides are common features in the vicinity of volcanic islands. In this contribution, we investigate landslides emplacement and dynamics around the volcanic island of Martinique based on the first scientific drilling of such deposits. The evolution of the active Montagne Pelée volcano on this island has been marked by three major flank-collapses that removed much of the western flank of the volcano. Subaerial collapse volumes vary from 2 to 25 km3 and debris avalanches flowed into the Grenada Basin. High-resolution seismic data (AGUADOMAR – 1999, CARAVAL – 2002 and GWADASEIS – 2009) is combined with new drill cores that penetrate up to 430 m through the three submarine landslide deposits previously associated to the aerial flank-collapses (Site U1399, Site U1400, Site U1401, IODP Expedition 340, Joides Resolution, March-April 2012). This combined geophysical and core data provide an improved understanding of landslide processes offshore a volcanic island. The integrated analysis shows a large submarine landslide deposit, without debris avalanche deposits coming from the volcano, comprising up to 300 km3 of remobilized seafloor sediment that extends for 70 km away from the coast and covers an area of 2100 km2. Our new data suggest that the aerial debris avalanche deposit enter the sea but stop at the base of submarine flank. We propose a new model dealing with seafloor sediment failures and landslide propagation mechanisms, triggered by volcanic flank-collapse events affecting Montagne Pelée volcano. Newly recognized landslide deposits occur deeper in the stratigraphy, suggesting the recurrence of large-scale mass-wasting processes offshore the island and thus, the necessity to better assess the associated tsunami hazards in the region. This article is protected by copyright. All rights reserved

A 1.5 Ma Marine Record of Volcanic Activity and Associated Landslides Offshore Martinique (Lesser Antilles): Sites U1397 and U1399 of IODP 340 Expedition

The products of eruptive and mass-wasting processes that built island arc volcanoes are better preserved in marine deposits than on land. Holes U1397A and U1399A drilled during IODP Expedition 340 provide a 1.5 Ma record of the volcanic history of Martinique. 14C dating and δ18O patterns are used to reconstitute the chronostratigraphy of tephra, volcaniclastic turbidites, and mass-wasting events (traced by debris avalanches, debrites, and duplication and deformation of pre-existing sediments), leading to a new volcanic history of Montagne Pelée and Pitons du Carbet volcanoes. The top 50 m of core U1397A provides a continuous high-resolution sedimentation record over the last ∼130 ka. The sedimentation record deeper than 50 m in core U1397A and in the whole core U1399A is discontinuous because of the numerous sliding and deformation events triggered by debris avalanches related to flank collapses. Three successive activity periods are identified since ∼190 ka: the “Old Pelée” until 50 ka, the “Grand Rivière” (50–20 ka), and the “Recent Pelée” (20 ka—present day). The first two periods have the highest volcanic deposition rates offshore but very little outcrop on land. The whole magmatic activity of Mt Pelée comprises silicic andesites, but mafic andesites were also emitted during the whole “Grand Rivière.” At ∼115 ka, a major flank collapse (“Le Prêcheur”) produced a debris avalanche and submarine landslide that affected sea floor sediments by erosion and deformation up to ∼70 km from the shore. The Pitons du Carbet volcano was active from 1.2 Ma to 260 ka with numerous large flank collapses at a mean rate of 1 event every 100 ka. The average deposition rate of tephra fall offshore is much less than that at Mt Pelée. Our data show that correlations between the timing of large landslides or emission of mafic magmas and rapid sea level rise or lowstands suggested by previous studies are not systematic. The reconstituted chronostratigraphy of cores U1397A and U1399A provides the framework necessary for further studies of the magma petrology and production rates and timing of the mechanisms triggering flank collapses and related submarine landslides of Mt Pelée and Pitons du Carbet

The ALBACORE oceanographic cruise: tectonic and sedimentary processes at distinct temporal and spatial scales in the Alboran Sea

European Geosciences Union (EGU) General Assembly, 23-27 May 2022, Vienna, AustriaThe Alboran Sea (Western Mediterranean) is a relatively small ocean basin connected with the Atlantic that provides a rich archive of tectonic and sedimentary processes at distinct temporal and spatial scales during the Quaternary. Since the collisional boundary of the Eurasia-Nubia plates crosses the Alboran Sea, this basin is also the locus of active geohazards: the constant seismic activity, concentrated mostly along the Al Idrissi strike-slip fault system and submarine landslides, that can cause tsunami hazards affecting the entire Alboran coasts and damages to submarine cables and infrastructures. Previous understanding of the Alboran Sea has been based on seafloor and subsurface geophysical data of differing resolution and scale, combined with very short sediment coring and IODP and industrial boreholes. In order to obtain new constrains on the geology of the Alboran Sea, the ALBACORE cruise was held in October and November 2021 onboard the R/V Pourquoi Pas? In addition to sites in the northern Alboran Sea targeting contourites, several sites in the southern Alboran Sea were selected as key study areas: the Al-Idrissi active fault zone, the Al-Hoceima shelf, the Xauen/Tofiño and the Francesc Pages banks. The scientific work of the ALBACORE campaign included the acquisition of Calypso cores (up to 28m long), sampling of consolidated strata with Cnexoville, in situ geotechnical measurements (Penfeld) with a seabed cone penetration test device (up to 50m long), heat flow measurements (up to 6m long), swath bathymetric imaging of the seafloor and water column, and sub-bottom profiling. The total length of sediments recovered reached 734m. Results from the ALBACORE cruise address the following scientific objectives: - To understand better the causal relationships between the present-day morpho-structural pattern and date Quaternary tectonic pulse and associated sedimentary systems - To determine the Late Pleistocene-Holocene stratigraphic pattern and the paleo-oceanographic implications of contourites. - To explore the chronological evolution of cold-water coral mounds and their paleoceanographic and palaeoclimatic signature since the Middle Pleistocene. - To investigate the causal factors of slope instability processes and evaluate the geological hazard associated with tectonic pulses and fluid seepage. - To determine the recent high-resolution sequence stratigraphy of the Al-Hoceima shelf in order to decode the late Pleistocene and Holocene sea-level changes at millennial scalePeer reviewe