Influence of Nb-doping on the structural and electrical properties of lanthanum molybdates, La5.4MoO11.1

Nowadays, hydrogen is receiving a great deal of attention as an energy carrier. Commonly, it is obtained by hydrocarbons reforming, such as natural gas, coal and biomass. However, the resulting hydrogen needs to be purified to remove by-products and impurities, increasing the production costs. An alternative for hydrogen production is proton-conducting ceramics, where hydrogen separation takes place via a chemical potential gradient across the membrane.1, 2 In this work, Nb-doped La6MoO12--based compounds have been investigated as part of a new family of materials very competitive as SOFC electrolyte and hydrogen separation membranes.3 These materials, La5.4Mo1-xNbxO11.1-x/2 (x = 0.05, 0.10, 0.15 y 0.20) were synthesized by the freeze-drying precursor method and calcination conditions have been optimized to obtain single phases. A complete characterization has been carried out using X-Ray powder diffraction and scanning and transmission electron microscopy. The total conductivity was determined by complex impedance spectroscopy at different atmospheres. Different polymorphs are obtained as a function of the cooling rate and the dopant amount. The samples cooled by quenching are cubic with a fluorite-type structure (Fm3 ̅m) and the ones cooled at 50 y 0.5 ºC•min-1 are rhombohedral (R1 and R2 polymorphs). For niobium contents higher than x = 0.10 the R1 polymorph is stabilised at cooling rates equal or inferior to 50 ºC•min-1. For all three series, the incorporation of niobium into La5.4MoO11.1 increases the conductivity, reaching the best values for x=0.10 and the sample obtained by quenching.Universidad de Málaga. Campus de Excelencia Internacional Andalucía Tech

Effect of preparation conditions on the polymorphism and transport properties of lanthanum molybdates

In this work, La6MoO12-based compounds were investigated as part of a new family of materials very competitive as hydrogen separation membranes [1,2]. La5.4MoO11.1 was synthesized by the freeze-drying precursor method and the calcination conditions were optimized in order to obtain single phases. Several cooling rates were applied and different polymorphs were obtained: a simple cubic fluorite symmetry (Fm-3m) for the sample cooled by quenching, and two different rhombohedral (R-3) space groups for the samples cooled at 50 ºC•min-1 and 0.5 ºC•min-1 (see Figure below). For the quenched sample, the Rietveld refinement was satisfactory in a Fm-3m space group. For the other two compositions no structural model was available and were indexed in a R-3 space group, however some small reflections were not given any intensity by the model used. Transmission electron microscopy confirmed the presence of superstructures for those samples. All ceramic materials were obtained with relative densities close to 100% after sintering at 1500 ºC. Stability studies demonstrated that all three polymorphs were stable in oxidizing and reducing conditions at 800 ºC for 48 hours. The three samples present a significant proton contribution to the conductivity at temperatures lower than 800 ºC. These results were confirmed by thermogravimetric analysis. The highest conductivity values were observed for the samples prepared by quenching. The three polymorphs display a small p-type electronic contribution to the overall conductivity in oxidizing conditions and n-type electronic one in very reducing conditions, much more significant for the samples cooled by quenching and at 50 ºC•min-1.Universidad de Málaga. Campus de Excelencia Internacional Andalucía Tech

Ajuste de las propiedades estructurales de molibdatos de lantano a través de la modificación de los entramados catiónicos y aniónicos

Los materiales basados en molibdatos de lantano, con fórmula general La6-xMoO12-, son candidatos a membranas de separación de hidrógeno debido su conductividad electrónica y protónica. Estos cristalizan en diferentes estructuras en función de la velocidad de enfriamiento, estructuras cúbicas si son enfriados súbitamente, o estructuras romboédricas si lo son a 50 o 0.5 ºC•min-1 [1]. Por otro lado, recientemente ha sido descubierto que la sustitución de Mo por Nb lleva a la estabilización de polimorfos de mayor simetría. Aparte de la aproximación tradicional de modificar la red catiónica de un material para mejorar sus propiedades, recientemente se está actuando sobre la subred de oxígenos, sustituyéndolos por átomos de flúor. Esta estrategia ha sido implementada con éxito en materiales tipo perovskita, con mejoras en los flujos de permeabilidad de oxígeno [2], o en materiales tipo La2Mo2O9, con modificaciones en sus propiedades de transporte [3]. En este trabajo se ha procedido a la modificación simultánea de la red catiónica y aniónica, a través de la incorporación de niobio y flúor, respectivamente, en molibdatos de lantano, con fórmula general La5.4Mo1-xNbxO12-y-Fy. Esta estrategia combinada da lugar a la fase de más alta simetría a baja temperatura (Figura 1).Universidad de Málaga. Campus de Excelencia Internacional Andalucía Tech

Effect of the modification of the cationic and anionic framework on the structural and electrical properties of lanthanum molybdates

Fluorinated lanthanum molybdates have been prepared at 1500 and 1200 ºC for 1 h and cooled down at 5 ºC min-1. The combination of anionic (F-) and cationic (Nb5+) doping stabilizes the cubic polymorph without needing a sudden cooling of the samples, increases the stability in reducing conditions and en-hances the electrical properties.Universidad de Málaga. Campus de Excelencia Internacional Andalucía Tec

Synthesis and characterization of M(II) phosphonates (M = Fe, Co, Zn, Mn) as precursors for PEMFCs electrocatalysts

Metal phosphonates are promising precursors for applications such as proton conductivity [1] and catalysis [2]. Specifically, upon calcination metal polyphosphates are generated that can be used as non-noble metal alternatives [3] to the highly expensive commercial catalysts (Pt) for proton exchange membrane fuel cells (PEMFCs). In this work, we present the synthesis and characterization of metal polyphosphates obtained from transition divalent metal phosphonates (M= Fe, Mn, Co and Zn) both as monometallic and bimetallic systems (solid solutions). For the preparation of the metal phosphonate precursors, two types of organic linkers were selected, i.e. 2-R,S-hydroxiphosphonoacetic acid [HO3PCH(OH)COOH, HPAA] and nitrilotrismethylenephosphonic acid [N(CH2PO3H2)3, ATMP]. The as synthesized compounds were calcined between 700 and 1000 ºC under N2. Depending on the metal/phosphorous molar ratio in the precursor phases, different compositions were found, the corresponding metal pyrophosphate being the major component according to the crystallographic data. Interestingly, in most of cases the solid solutions were preserved in the final product, for instance Fe-Mn, Fe-Co and Fe-Zn. All calcined materials have been also characterized by XPS, SEM/EDS, FTIR-Raman.Universidad de Málaga. Campus de Excelencia Internacional Andalucía Tech

Metal-Doping of La5.4MoO11.1 Proton Conductors: Impact on the Structure and Electrical Properties.

La5.4MoO11.1 proton conductors with different metal doping (Ca2+, Sr2+, Ba2+, Ti4+, Zr4+, and Nb5+) have been prepared and structurally and electrically characterized. Different polymorphs are stabilized depending on the doping and cooling rate used during the synthesis process. The most interesting results are obtained for Nb-doping, La5.4Mo1−xNbxO11.1−x/2, where single compounds are obtained in the compositional range 0 ≤ x ≤ 0.2. These materials are fully characterized by structural techniques such as X-ray and neutron powder diffraction and transmission electron microscopy, which independently confirm the changes of polymorphism. Scanning electron microscopy and impedance spectroscopy measurements in dry/wet gases (N2, O2, and 5% H2−Ar) showed an enhancement of the sinterability and electrical properties of the materials after Nb-doping. Conductivity measurements under very reducing conditions revealed that these materials are mixed ionic-electronic conductors, making them potential candidates for hydrogen separation membranes

Synergic Effect of Metal and Fluorine Doping on the Structural and Electrical Properties of La5.4MoO11.1-Based Materials.

Cationic and anionic frameworks of La5.4MoO11.1 proton conductors have been modified by means of metal (Ti4+, Zr4+, and Nb5+) and fluorine (F−) doping. This synergic effect leads to the stabilization of highsymmetry and single-phase polymorphs. The materials have been fully characterized by structural techniques, such as X-ray and neutron powder diffraction and transmission electron microscopy. The fluorine content was determined by ion chromatography. Impedance spectroscopy analysis under different atmospheres (dry and wet N2 and O2 and wet 5% H2−Ar) showed an improvement in the electronic conductivity under reducing conditions, making these materials potential candidates for hydrogen separation membranes

La tercera revolución industrial en México: diagnóstico e implicaciones

Los materiales que se ofrecen en este volumen fueron compilados por los organizadores del Simposio Multidisciplinario sobre La tercera revolución industrial en México: diagnóstico e implicaciones socioculturales, económicas y científico-tecnológicas, que se realizó del 30 de septiembre al 4 de octubre de 1991 en el Auditorio Mario de la Cueva, en la Torre 11 de Humanidades de la Universidad Nacional Autónoma de México (UNAM). Esta reunión surgió de la propuesta de los investigadores Ma. Luisa Rodríguez-Sala, del Instituto de Investigaciones Sociales, y de Adrián Chavero, delInstituto de Investigaciones Económicas de la UNAM con el objeto de abordar la problemática de la ciencia y la tecnología desde una perspectiva multidisciplinaria y consecuentemente interinstitucional. Se juzgó conveniente que con base en el planteamiento elaborado para convocar a los ponentes se formaran tres apartados temáticos que incluyeran a grandes rasgos los temas siguientes: 1) Un marco epistemológico que permitiera reflexionar sobre el contenido y significado de la tercera revolución industrial y su alcances sociales. 2) El análisis de los elementos de diagnóstico en cuanto a políticas y toma de decisiones para conocer de qué manera empieza el país a enfrentarse a esa situación novedosa, y 3) Un diagnóstico del nivel específico de la investigación del país en el tema central visualizado desde la perspectiva de los científicos y los tecnólogos involucrados directamente en el campo de la investigación en algunas de las disciplinas que ya forman parte de la denominada tercera revolución industrial, la cual, en términos generales, se puede caracterizar por: a) La creación de sucedáneos a las materias primas que serán desplazadas por aquellos "nuevos materiales" producidos en laboratorio a escala industrial. b) La introducción de la informática, que entre otras consecuencias puede revolucionar el sistema educativo. c) La introducción al mercado de los productos que provienen de la biotecnología y la agroindustria, ya logrados en los países de elevado nivel de desarollo. Puede considerarse que se cuenta con un diagnóstico muy completo de la situación del país en esta materia,que abarca no sólo el aspecto científico-tecnológico sino también la repercusión en los ámbitos económicos; sociales y culturales que se tocan en estos trabajos. Sin embargo, se requiere la revisión periódica y recurrente de lo que significa para México el impacto de la revolución industrial, sobre todo su reflexión en marcos o situaciones que de aquí en adelante se presentarán cada vez más dinámicos, cambiantes y novedosos, con el objetivo de apuntar hacia soluciones a la problemática que plantea para México la irrupción de la tercera revolución industrial

Transverse. Art projection space a public that is trained in art maybe a less violent public

Este articulo recopila algunos testimonios de varios integrantes que consolidan en palabras sus apreciaciones más significativas de estos cuatro años de estar presentes en el proyecto Espacio Transverso, del grupo de investigación TEI (Teatro espacio & interactividad) y su semillero llamado de la misma forma TEI. Compartiendo con los lectores fragmentos de reflexiones del proceso vivencial y su importancia en la proyección artística de la ciudad, Facultad de Bellas Artes en la Universidad del Atlántico, premisas a partir de la búsqueda por los integrantes y su principal impacto en la académica y en el arte; como proyecto de proyección y extensión hacia la comunidad del caribe colombiano para generar público y espacialmente mirarlo desde la práctica de circulación hecho que para los artistas es fundamental en el ámbito académico entre estudiantes, docentes y egresados para el transcurso del proyecto entre teoría y práctica artística para un público menos violento .This article collet some testimonials from several members that consolidate in words their appreciations most significant from these four years of being present in Espacio Transverso project, from investigation group TEI (Theater, Space & Interactivity) and its seedbed called equally TEI.Sharing with the readers fragments of reflections of the vivencial project and its importance in the artistic projection of the city, Facultad de Bellas Artes in Universidad del Atlántico, premises from the search by the integrants and their main impact in the academy and in art; as projection project and extension towards community of the Colombian caribbean to generate audience and especially view it since the practice of circulation, a fact that for artists is fundamental in the academic field among students, teachers and graduates for the course of the project between theory and artistic practice for a less violent audience

Single and solid solutions metal carboxyphosphonates as precursors for phosphorous-containing electrocatalysts

Co1-xFex-HPAA metal phosphonates were prepared and pyrolyzed under N2 and H2/Ar flows. The result-ing powders were structural and electrochemically characterised by XRD and CV, respectively. The ob-tained materials possess different structures depending on the temperature and pyrolysis atmosphere. Samples pyrolized in N2 at 1000 ºC are monophasic diphosphates, while phosphides form in H2/Ar at 850 ºC. Cyclic voltammetry studies indicate that the Co0.5Fe0.5HPAA sample, pyrolized in H2/Ar, exhibits the best behaviour, but further studies are needed to fully understand the role of graphene oxide and pyrazine.Universidad de Málaga. Campus de Excelencia Internacional Andalucía Tec