The Zintl-Klemm concept applied to cations in oxides. I. The structures of ternary aluminates

The structures of 94 ternary aluminates are reinterpreted on the basis of the Zintl-Klemm concept and Pearson's generalized octet rule. In aluminates of highly electropositive metals such as alkali, alkaline-earth and rare-earth metals, the Al atoms form three-dimensional skeleta which can be interpreted as if the Al atoms were behaving as Zintl polyanions, adopting the structure of either main-group elements or Zintl polyanions showing the same connectivity. The O atoms are then located close to both the hypothetical two-electron bonds and the lone pairs, giving rise to a tetrahedral coordination. When more electronegative elements, such as W or Si, are present in the compound, the electron transfer towards the Al atoms does not take place. In this case, aluminium behaves as a base, transferring its electrons to the more electronegative atoms and the coordination sphere of aluminium becomes octahedral. In some compounds the Al atoms clearly show amphoteric character so that some Al atoms act as donors (bases) and hence are octahedrally coordinated, whereas others behave as acceptors (acids), adopting a tetrahedral coordination. From this it is concluded that the coordination sphere of aluminium is not a function of the ionic radius of the Al3+ cations, but it depends on the nature of the other cations accompanying them in the structure. The networks formed by these aluminates are, in many instances, similar to those of the binary oxides of the main-group elements. For this reason, a systematic survey of these oxides is also reported. Compounds such as stuffed cristobalites and trydimites and also perovskites are examples of this new interpretation. Perovskites are then reinterpreted as a stuffed pseudo-TeO3 structure. Other families of compounds such as silicates and phosphates are susceptible to a similar interpretation. This study provides additional examples of how cations recognize themselves in spite of being embedded in an oxygen matrix.[to be completed]Peer reviewe

The Zintl-Klemm concept applied to cations in oxides. I. The structures of ternary aluminates

The structures of 94 ternary aluminates are reinterpreted on the basis of the Zintl-Klemm concept and Pearson's generalized octet rule. In aluminates of highly electropositive metals such as alkali, alkaline-earth and rare-earth metals, the Al atoms form three-dimensional skeleta which can be interpreted as if the Al atoms were behaving as Zintl polyanions, adopting the structure of either main-group elements or Zintl polyanions showing the same connectivity. The O atoms are then located close to both the hypothetical two-electron bonds and the lone pairs, giving rise to a tetrahedral coordination. When more electronegative elements, such as W or Si, are present in the compound, the electron transfer towards the Al atoms does not take place. In this case, aluminium behaves as a base, transferring its electrons to the more electronegative atoms and the coordination sphere of aluminium becomes octahedral. In some compounds the Al atoms clearly show amphoteric character so that some Al atoms act as donors (bases) and hence are octahedrally coordinated, whereas others behave as acceptors (acids), adopting a tetrahedral coordination. From this it is concluded that the coordination sphere of aluminium is not a function of the ionic radius of the Al3+ cations, but it depends on the nature of the other cations accompanying them in the structure. The networks formed by these aluminates are, in many instances, similar to those of the binary oxides of the main-group elements. For this reason, a systematic survey of these oxides is also reported. Compounds such as stuffed cristobalites and trydimites and also perovskites are examples of this new interpretation. Perovskites are then reinterpreted as a stuffed pseudo-TeO3 structure. Other families of compounds such as silicates and phosphates are susceptible to a similar interpretation. This study provides additional examples of how cations recognize themselves in spite of being embedded in an oxygen matrix.[to be completed]Peer reviewe

Tuning the propagation constant by the anti-crossing bandgap prism coupling technique.

A novel plasmonic structure based on an anticrossing bandgap prism coupling technique is proposed. The study has been carried out using photonic crystals based on diffraction gratings (bounded by dielectrics with identical dielectric functions) together with a high refractive index prism to couple the long-range surface plasmon polaritons to photons. We analyse the structure and demonstrate the ability for tuning the propagation constants of plasmon modes by changing the thickness of the gold grating. The comparison to non-bandgap techniques is studied, and the influence of the plasmonic configuration on the plasmon propagation constant is discussed as well. Experimental measurements were also carried out to confirm the validity of our model

Análisis energético y exergético de un motor diesel de automoción operando en diferentes altitudes

RESUMEN: La densidad del aire disminuye con el aumento de la altitud sobre el nivel del mar, este aspecto afecta el proceso de combustión, la formación de emisiones contaminantes y por tanto el desempeño del motor. En este trabajo se presenta el diagnóstico del proceso de combustión de un motor diesel de automoción turbo-alimentado, mediante la medición de presión en cámara operando en tres alturas diferentes sobre el nivel del mar, bajo condiciones estacionarias, utilizando diesel convencional (acpm) como combustible. A medida que aumenta la altura sobre el nivel del mar se incrementa la relación combustible/aire (mezcla más rica) y con ello el consumo específico de combustible, la duración de la combustión, la combustión en fase premezclada, la temperatura máxima, el calor transferido a los gases y la exergía destruida, mientras que el rendimiento térmico efectivo del motor, la presión máxima y la exergía en el cilindro disminuyen. Sin embargo, la eficiencia mecánica y el tiempo de inyección se mantienen aproximadamente constantes. Las diferencias encontradas en la exergía destruida se deben a las variaciones del proceso de combustión, ya que no se encontraron efectos significativos en las carreras de compresión y expansión. La mayor irreversibilidad debida al aumento de la altura se debe a la baja calidad de la energía de los gases de escape.ABSTRACT: Altitude above sea level produces a reduction in air density affecting the combustion process, pollutant emissions, and engine performance. In this work the combustion diagnosis of an automotive turbocharged diesel engine was carried out from in-cylinder pressure signal. Tests were performed at three altitudes above sea level, under steady state operating conditions, using conventional diesel fuel. As altitude above sea level increased, the fuel/air mixture became richer. The brake specific fuel consumption, combustion duration, premixed combustion phase, maximum temperature, heat rejected to the gases and exergy destruction were also increased; at the same time, brake thermal efficiency, maximum in-cylinder pressure and in-cylinder exergy decreased. Mechanical efficiency and injection timing remained approximately invariable. Exergy destruction differences were caused by the combustion process, without significant effects during compression and expansion. The greater irreversibility resulting from altitude increase was linked with the lower energy quality of the exhaust gases

Oxidation of High Yield Strength Metals Tungsten and Rhenium in High-Pressure High-Temperature Experiments of Carbon Dioxide and Carbonates

The laser-heating diamond-anvil cell technique enables direct investigations of materials under high pressures and temperatures, usually confining the samples with high yield strength W and Re gaskets. This work presents experimental data that evidences the chemical reactivity between these refractory metals and CO2 or carbonates at temperatures above 1300 °C and pressures above 6 GPa. Metal oxides and diamond are identified as reaction products. Recommendations to minimize non-desired chemical reactions in high-pressure high-temperature experiments are given

Conceptual analysis of influential factors in the motivation and involvement of the university student towards the assessment in physical education

The involvement of university students in their own assessment leads to motivation towards learning. The aim of this study is twofold. The first aim is to analyze the factors of evaluation in Physical Education that generate motivation and involvement. To do this, we have discussed five factors: (1) to generate awareness of what is being learned; (2) to delimit the deadlines in the development of tasks; (3) to associate it with the development of competencies; (4) to apply it in a variety of contexts and tasks; and (5) to allow reflection on the teaching and learning process. The second aim, based on the previous one, tries to delimit strategies to implement a quality evaluation in the university classrooms. Five strategies have been analyzed: (1) triadic assessment; (2) grade distribution; (3) collaborative creation of instruments; (4) elaboration of formative questions; and (5) use of apps. Finally, a series of reflections are proposed to put into practice the formative and shared assessment as opposed to the traditional evaluation. This manuscript is a substantial contribution to the existing literature, as it serves as a clear guide for university teachers to implement this type of assessment in the classroom, generating student motivation and transferability of their learning

Conceptual Analysis of Influential Factors in the Motivation and Involvement of the University Student towards the Assessment in Physical Education

[EN] The involvement of university students in their own assessment leads to motivation towards learning. The aim of this study is twofold. The first aim is to analyze the factors of evaluation in Physical Education that generate motivation and involvement. To do this, we have discussed five factors: (1) to generate awareness of what is being learned; (2) to delimit the deadlines in the development of tasks; (3) to associate it with the development of competencies; (4) to apply it in a variety of contexts and tasks; and (5) to allow reflection on the teaching and learning process. The second aim, based on the previous one, tries to delimit strategies to implement a quality evaluation in the university classrooms. Five strategies have been analyzed: (1) triadic assessment; (2) grade distribution; (3) collaborative creation of instruments; (4) elaboration of formative questions; and (5) use of apps. Finally, a series of reflections are proposed to put into practice the formative and shared assessment as opposed to the traditional evaluation. This manuscript is a substantial contribution to the existing literature, as it serves as a clear guide for university teachers to implement this type of assessment in the classroom, generating student motivation and transferability of their learning.S

Biomass gasification process: theoretical and experimental studies a review

RESUMEN: La situación energética y medioambiental mundial hace que la generación de energía mediante sistemas basados en energías renovables tome cada vez una mayor relevancia. Estos sistemas tienen una doble ventaja ya que posibilitan la diversificación energética y reducen la dependencia del petróleo a la vez que se disminuyen las emisiones globales de CO2 La biomasa es uno de los recursos energéticos con mayor potencial tanto en países desarrollados como emergentes pero sigue sin sufrir el desarrollo esperado. La gasificación de biomasa es uno de los sistemas con gran potencial ya que permite la generación tanto de energía térmica como eléctrica, mediante tecnologías de bajo costo como los gasificadores de lecho fijo (potencias medias – bajas), o reactores de lecho fluidizado con tecnología más exigente y con mayor capacidad de generación. La transformación de la biomasa en un combustible gaseoso, aprovechable en motores de combustión interna alternativos o en turbinas de gas como combustible, exige que la biomasa atraviese varios procesos termoquímicos, compuestos por reacciones endotérmicas (secado, devolatilización y reducción) y exotérmicas (oxidación de volátiles y carbón). La selección de un gasificador requiere conocer las propiedades de la biomasa, mientras que el diseño del mismo exige conocer el fenómeno termoquímico por completo. El objetivo de este trabajo es presentar una revisión actualizada de los diferentes planteamientos tanto teóricos como experimentales desarrollados para estudiar el proceso de gasificación de biomasa, tanto en lecho fijo como en lecho fluidizado, con miras a diseñar gasificadores de biomasa.ABSTRACT: Each day the energy systems relying on renewable resources are more relevant in the world energy politics due to the fossil energy and environmental crisis. These systems increase energy resources of countries, reduce the oil dependency, and diminish CO2 global emissions. Biomass is one of the renewable resources with higher potential in developing and developed countries. Biomass gasification has huge possibilities to be developed. Such as fixed bed gasifiers, or fluidized bed reactors with more technology demanding and more power capacity. Biomass transformation in a gaseous fuel to be used in a reciprocating internal combustion engine or gas turbines requires that biomass passes through several thermochemical stages. Endothermic reactions (drying, pyrolysis and reduction) and exothermic process (volatiles and carbon oxidation). To select a specific gasifier it is necessary to know biomass properties (chemical and physical), whereas to design a reactor it is necessary to know and understand the complete phenomena. The aim of this work is to show an actualized review about different theoretical and experimental approaches to study biomass gaification process in fixed and fluidized bed reactors in order to design biomass gasifiers

Metastable structural transformations and pressure-induced amorphization in natural (Mg,Fe)2SiO4 olivine under static compression: A Raman spectroscopic study

[EN] Raman spectroscopic data were obtained for (Mg,Fe)(2)SiO4 samples during compression to 57 GPa. Single crystals of San Carlos olivine compressed hydrostatically above 41 GPa showed appearance of a new "defect" peak in the 820-840 cm(-1) region associated with SiOSi linkages appearing between adjacent SiO44- tetrahedra to result in five- or sixfold-coordinated silicate species. Appearance of this feature is accompanied by a broad amorphous background. The changes occur at lower pressure than metastable crystalline transitions of end-member Mg2SiO4 forsterite (Fo-I) into Fo-II and Fo-III phases described recently. We complemented our experimental study using density functional theory (DFT) calculations and anisotropic ion molecular dynamics (AIMD) simulations to study the Raman spectra and vibrational density of states (VDOS) of metastably compressed Mg2SiO4 olivine, Fo-II and Fo-III, and quenched melts at high and low pressures. By 54 GPa all sharp crystalline peaks disappeared from observed Raman spectra indicating complete pressure-induced amorphization (PIA). The amorphous (Mg,Fe)(2)SiO4 spectrum contains Si-O stretching bands at lower wavenumber than expected for SiO44- indicating high coordination of the silicate units. The amorphous spectrum persisted on decompression to ambient conditions but with evidence for reappearance of tetrahedrally coordinated units. Non hydrostatic compression of polycrystalline olivine samples showed similar appearance of the defect feature and broad amorphous features between 43-44 GPa. Both increased in intensity as the sample was left at pressure overnight but they disappeared during decompression below 17 GPa with recovery of the starting olivine Raman signature. A hydrated San Carlos olivine sample containing 75-150 ppm OH was also studied. Significant broadening of the SiO44- stretching peaks was observed above 43 GPa but without immediate appearance of the defect or broad amorphous features. However, both of these characteristics emerged after leaving the sample at 47 GPa overnight followed by complete amorphization that occurred upon subsequent pressurization to 54 GPa. During decompression the high-density amorphous spectrum was retained to 3 GPa but on final pressure release a spectrum similar to thermally quenched low-pressure olivine glass containing isolated SiO44- groups was obtained. Leaving this sample overnight resulted in recrystallization of olivine. Our experimental data provide new insights into the metastable structural transformations and relaxation behavior of olivine samples including material recovered from meteorites and laboratory shock experiments.Our work was supported by the U.K. NERC via Grant NE/K002902/1 and Spanish MINECO under projects MAT2014-46649-C4-1/2-P.Santamaría Pérez, D.; Thomson, A.; Segura, A.; Pellicer Torres, J.; Manjón, F.; Cora, F.; Mccoll, K.... (2016). Metastable structural transformations and pressure-induced amorphization in natural (Mg,Fe)2SiO4 olivine under static compression: A Raman spectroscopic study. American Mineralogist. 101(7):1642-1650. https://doi.org/10.2138/am-2016-5389CCBYS16421650101

Reversibility of the zinc-blende to rock-salt phase transition in cadmium sulfide nanocrystals

CdS nanoparticles prepared by a mechanochemical reaction in a planetary ball mill have been investigated by x-ray diffraction, optical absorption, and Raman scattering under high pressure conditions up to 11 GPa. The zinc-blende (ZB) to rock-salt phase transition is observed around 6 GPa in all experiments, the transition pressure being similar to the one measured in CdS colloidal nanocrystals, and much higher than in bulk (around 3 GPa). The direct optical energy gap in ZB-CdS increases with pressure, and suddenly drops when the pressure is raised above 6 GPa, according to the high-pressure indirect-gap behavior. A linear blue-shift of the CdS Raman spectra is observed upon increasing pressure. Both Raman and x-ray diffraction studies indicate that the phase transition has a large hysteresis, making the ZB phase barely recoverable at ambient conditions. Cell parameters and bulk modulus measured in CdS nanoparticles clearly show that the nanoparticles at ambient conditions are subject to an initial pressure in comparison to CdS bulk.This work was financially supported by the Spanish Ministerio de Ciencia e Innovación (Projects MAT2008-06873-C02-01/MAT, MAT2011-28868-C02-01 and CTQ2009-14596-C02-01), the Comunidad de Madrid and European Social Fund (Project S2009/PPQ-1551 4161893 (QUIMAPRES)), and the MALTA-Consolider Ingenio 2010 (Reference CSD2007-00045). R.M.-R. thanks the Spanish MEC for a FPI research grant (Reference BES-2006-13359). The expert assistance of L. Rodríguez at the SERTEM is gratefully acknowledged