Person: Morán Miguélez, Emilio
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First Name
Emilio
Last Name
Morán Miguélez
Affiliation
Universidad Complutense de Madrid
Faculty / Institute
Ciencias Químicas
Department
Química Inorgánica
Area
Química Inorgánica
Identifiers
12 results
Search Results
Now showing 1 - 10 of 12
Item Microwave-assisted synthesis: A fast and efficient route to produce LaMO3 (M=Al, Cr, Mn, Fe, Co) perovskite materials(Materials Research Bulletin, 2011) Prado Gonjal, Jesús De La Paz; Arévalo-López, Ángel Moisés; Morán Miguélez, EmilioA series of lanthanum perovskites, LaMO3 (M=Al, Cr, Mn, Fe, Co), having important technological applications, have been successfully prepared by a very fast, inexpensive, reproducible, environment-friendly method: the microwave irradiation of the corresponding mixtures of nitrates. Worth to note, the microwave source is a domestic microwave oven. In some cases the reaction takes place in a single step, while sometimes further annealings are necessary. For doped materials the method has to be combined with others such as sol–gel. Usually, nanopowders are produced which yield high density pellets after sintering. Rietveld analysis, oxygen stoichiometry, microstructure and magnetic measurements are presented.Item The role of defects in microwave and conventionally synthesized LaCoO3 perovskite(2016) Prado Gonjal, Jesús De La Paz; Gutiérrez-Seijas, Julia; Herrero Ansorregui, Irene; Morán Miguélez, Emilio; Terry, Ian; Schmidt, RainerIn this work we investigate the magnetic, dielectric and charge transport properties of LaCoO3 (LCO) synthesized by two different techniques: microwave assisted and conventionally heated ceramic synthesis. The rapid microwave synthesis conditions are far away from thermodynamic equilibrium and are found to lead to modified crystal defect properties as compared to conventional synthesis. The thermally induced magnetic spin state transition at Ts1 ≈ 80 K is exemplified by temperature (T)-dependent dielectric spectroscopy data, which reveal the appearance of an additional dielectric contribution that is correlated to the transition. Magnetisation, M vs T, and electrical resistivity, ρ vs T, curves show that the additional dielectric phase is strongly influenced by magnetic defects and may be associated with higher spin state clusters in a magnetic spin-state coexistence scenario. We suggest that defects such as oxygen vacancies act as magnetic nucleation centres across the spin state transition Ts1 for the formation of higher spin state clusters in LCO perovskites.Item Anti-site disorder and physical properties in microwave synthesized RE2Ti2O7 (RE = Gd, Ho) pyrochlores(RSC Advances, 2015) Gómez-Pérez, Alejandro; Prado Gonjal, Jesús De La Paz; Muñoz Gil, Daniel; Andrada Chacón, Adrián; Sánchez Benítez, Francisco Javier; Morán Miguélez, Emilio; Azcondo, MaríaTeresa; Amador, Ulises; Schmidt, RainerIn this work we report on the microwave assisted synthesis of nano-sized Gd2Ti2O7 (GTO) and Ho2Ti2O7 (HTO) powders from the RE2Ti2O7 pyrochlore family (RE ¼ rare earth). Synchrotron X-ray powder diffraction was used to study RE–Ti cationic anti-site defects with concentrations that decrease in both samples with increasing temperature starting from 1100 C, and the defects disappear at 1400 ºC. SQUID magnetometry measurements revealed that GTO shows a predominantly anti-ferromagnetic structure, whereas HTO exhibits magnetic saturation and a ferromagnetic component at low temperature. Impedance spectroscopy data revealed strongly increased ionic oxygen vacancy conduction in HTO ceramic pellets as compared to GTO, which may be associated with a higher degree of oxygen vacancy disorder. This argument was supported by Raman spectroscopy data.Item Microwave-Assisted Synthesis, Microstructure, and Physical Properties of Rare-Earth Chromites(Inorganic chemistry, 2012) Prado Gonjal, Jesús De La Paz; Schmidt, Rainer; Romero, Juan-José; Ávila Brande, David; Amador, Ulises; Morán Miguélez, EmilioThe full rare-earth (RE) chromites series (RE)CrO3 with an orthorhombic distorted (Pnma) perovskite structure and the isostructural compound YCrO3 can be synthesized through a simple microwave-assisted technique, yielding high-quality materials. Magnetization measurements evidence that the Néel temperature for antiferromagnetic Cr3+–Cr3+ ordering strongly depends on the RE3+ ionic radius (IOR), and a rich variety of different magnetic spin interactions exists. Dielectric spectroscopy on sintered pellets indicates electronic inhomogeneity in all samples as manifested by the presence of at least two dielectric relaxation processes associated with grain boundary and grain interior bulk contributions. X-ray diffraction, Raman spectroscopy, and temperature-dependent dielectric permittivity data do not indicate potential noncentrosymmetry in the crystal or concomitant ferroelectricity. Strong correlations between the magnetic and dielectric properties were not encountered, and microwave-synthesized (RE)CrO3 may not be classified as magnetoelectric or multiferroic materials.Item Microwave synthesis & sintering of Sm and Ca co-doped ceria ceramics(International Journal of Hydrogen Energy, 2015) Prado Gonjal, Jesús De La Paz; Heuguet, Romain; Muñoz Gil, Daniel; Rivera Calzada, Alberto Carlos; Marinel, Sylvain; Morán Miguélez, Emilio; Schmidt, RainerIn this work we report on the combined use of microwave (MW) heating sources for the powdersynthesisandthe ceramic sinteringofSmandCaco-doped ceria Ce0.8Sm0.18Ca0.02O1.9 polycrystalline materials for potential application as an electrolyte in intermediatetemperature solid oxide fuel cells (IT-SOFCs). We investigate the crystal structure, ceramic microstructure and the oxygen ion conductivity in detail and compare the latter to conventionally sintered ceramics. MW sintering of ceramic pellets leads to only slightly increased resistivity as compared to conventional sintering, but offers massive energy and time savings for potential industrial production processes. Exceptionally high oxygen ion conductivity without any significant electronic contribution was found in MW synthesized and MW sintered pellets, where the total resistivity, composed of grain boundary and bulk contributions, was found to be in the range of 0.5-1 kohmios at 500 ºC. Sm- and Ca co-doped ceria may be well-suited for electrolyte materials in IT-SOFCs.Item From theory to experiment: BaFe0.125Co0.125Zr0.75O3−δ, a highly promising cathode for intermediate temperature SOFCs(Journal of Materials Chemistry A, 2020) Sánchez Ahijón, Elena; Marín Gamero, Rafael; Molero-Sánchez, Beatriz; Ávila Brande, David; Manjón-Sanz, Alicia; Fernández-Díaz, M. Teresa; Morán Miguélez, Emilio; Schmidt, Rainer; Prado Gonjal, Jesús De La PazIn a recent theoretical study [Jacobs et al., Adv. Energy Mater., 2018, 8, 1702708], BaFe0.125Co0.125Zr0.75O3−δ was predicted to be a stable phase with outstanding performance as an auspicious cathode for intermediate-temperature solid oxide fuel cells (IT-SOFCs). It is shown here that the theoretical predictions are valid. The material can be synthesized by the citrate method as a single cubic Pm[3 with combining macron]m phase with a significant amount of oxygen vacancies, randomly distributed in the anionic sublattice facilitating oxygen vacancy conduction. A thermal expansion coefficient of 8.1 × 10−6 K−1 suggests acceptable compatibility with common electrolytes. Electrochemical impedance spectroscopy of symmetrical cells gives an area-specific resistance of 0.33 Ω cm2 at 700 °C and 0.13 Ω cm2 at 800 °C. These values are reduced to 0.13 Ω cm2 at 700 °C and 0.05 Ω cm2 at 800 °C when the material is mixed with 30 wt% Ce0.9Gd0.1O2−δ.Item Structural and physical properties of microwave synthesized orthorhombic perovskite erbium chromite ErCrO3(Journal of the European Ceramic Society, 2012) Prado Gonjal, Jesús De La Paz; Schmidt, Rainer; Ávila Brande, David; Amador, Ulises; Morán Miguélez, EmilioRare-earth chromite ErCrO3 powder was synthesized from metal nitrate precursors using microwave synthesis. (Micro-) structural characterizations were performed using X-ray diffraction, Rietveld refinement, High Resolution Transmission Electron Microscopy and Electron Micro-Diffraction. Magnetization vs. temperature measurements revealed anti-ferromagnetism with TNeel ≈ 135 K. An anti-ferrimagnetic moment of ≈0.4 μB was determined from magnetization vs. applied field measurements. Temperature dependent impedance spectroscopy (IS) indicated 3 dielectric relaxation processes: electrode interface, grain boundary and bulk. The intrinsic bulk activation energy was found to be 0.27 eV and the dielectric permittivity ɛr was ≈23 in excellent agreement with Clausius–Mossotti predictions and showed no perceptible temperature dependence. This and the low ɛr value suggested that ErCrO3 is a dielectric rather than ferroelectric compound. IS measurements with applied dc bias revealed the signs of an unconventional type of Schottky barrier at the metallic Au electrode/ceramic ErCrO3 interface.Item Microwave-assisted synthesis and characterization of new cathodic material for solid oxide fuel cells: La0.3Ca0.7Fe0.7Cr0.3O3−d(Ceramics International, 2015) Molero-Sánchez, Beatriz; Prado Gonjal, Jesús De La Paz; Ávila Brande, David; Birss, Viola; Morán Miguélez, EmilioIn this work, we examine the benefits of alternative powder processing methods, with a primary focus on microwave-based synthesis, that could both lower material manufacturing costs and further enhance cathode performance for solid oxide fuel cell applications. La0.3Ca0.7Fe0.7Cr0.3O3−δ (LCFCr), formed using conventional solid-state methods, has been shown in earlier work to be a very promising catalyst for the oxygen reduction reaction. To further increase its performance, microwave methods were used to increase the surface area of LCFCr and to decrease the synthesis time. It was found that the material could be obtained in crystalline form in only 7 h, with the synthesis temperature lowered by roughly 300 °C as compared to conventional methods.Item Increased ionic conductivity in microwave hydrothermally synthesized rare-earth doped ceria Ce1−xRExO2−(x/2)(Journal of Power Sources, 2012) Prado Gonjal, Jesús De La Paz; Schmidt, Rainer; Espíndola-Canuto, Jesús; Ramos-Alvarez, Paola; Morán Miguélez, EmilioCe0.85RE0.15O1.925 (RE = Gd, Sm), Ce0.8(Gd0.1Sm0.1)O1.9 and CeO2−δ nano-powders were synthesized by microwave-assisted hydrothermal synthesis in a time and energy efficient way. The fluorite-type crystal structure of the synthesized nano-powders was confirmed by X-ray diffraction (XRD) and Rietveld refinement, and the nano-metric particle sizes calculated from the XRD line width broadening were consistent with transmission electron microscopy observations. The Brunauer–Emmett–Teller method was used to confirm large powder surface area. Scanning electron microscopy confirmed high density and low surface porosity of the sintered ceramics as a result of the high sintering activity of the large surface area nano-powders. Impedance spectroscopy was carried out to separately analyze grain boundary (GB) and bulk dielectric relaxations where GB areas were found to constitute ionic charge transport barriers, because their resistance was larger than that of the bulk. Such barriers were found to be comparatively low due to high GB and bulk ceramic ionic conductivities, with the highest values encountered in the composition Ce0.85Sm0.15O1.925. The optimal ionic conductivities encountered were associated with the effect of high sintering activity of the nano-powders.Item Lithium Intercalation Mechanism and Critical Role of Structural Water in Layered H2V3O8 High-Capacity Cathode Material for Lithium-Ion Batteries(Chemistry of Materials, 2022) Kuhn, Alois; Perez-Flores, Juan Carlos; Prado Gonjal, Jesús De La Paz; Morán Miguélez, Emilio; Hoelzel, Markus; Díez-Gómez, Virginia; Sobrados, Isabel; Sanz, Jesús; García Alvarado, FlavianoH2V3O8 (HVO) is a promising high-capacity cathode material for lithium-ion batteries (LIBs). It allows reversible two-electron transfer during electrochemical lithium cycling processes, yielding a very attractive theoretical capacity of 378 mAh g–1. While an abundant number of research works exclusively proved the outstanding electrochemical lithium storage properties of H2V3O8, structural changes during the intercalation process have not been scrutinized, and the crystallographic positions occupied by the guest species have not been revealed yet. However, an in-depth understanding of structural changes of cathode materials is essential for developing new materials and improving current materials. Aimed at providing insights into the storage behavior of HVO, in this work, we employed a combination of high-resolution synchrotron X-ray and neutron diffraction to accurately describe the crystal structures of both pristine and lithiated H2V3O8. In HVO, hydrogen is located on one single-crystallographic site in a waterlike arrangement, through which bent asymmetric hydrogen bonds across adjacent V3O82– chains are established. The role played by water in network stabilization was further examined by density functional theory (DFT) calculations. Easy hydrogen-bonding switch of structural water upon lithium intercalation not only allows better accommodation of intercalated lithium ions but also enhances Li-ion mobility in the crystal host, as evidenced by magic-angle spinning (MAS) NMR spectroscopy. Facile conduction pathways for Li ions in the structure are deduced from bond valence sum difference mapping. The hydrogen bonds mitigate the volume expansion/contraction of vanadium layers during Li intercalation/deintercalation, resulting in improved long-term structural stability, explaining the excellent performance in rate capability and cycle life reported for this high-energy cathode in LIBs. This study suggests that many hydrated materials can be good candidates for electrode materials in not only implemented Li technology but also emerging rechargeable batteries.