No estequiometria y estabilidad en agua de SrCeO3 no dopado

[EN] Strontium cerate is the parent phase of an important class of proton-conducting perovskites with various potential technological applications. Phase formation and structure of SrCeO3 with Sr:Ce nonstoichiometry have been investigated for the series, Sr1±xCeO3±δ (0.98 ≤ x ≤ 1.04). Analyses by EPMA (electron probe micro analysis) and X-ray diffraction (XRD) indicate that, for samples sintered at 1350°C, the main phase is Sr-rich for all x. The accommodation of excess SrO in the bulk phase and/or intergranular regions is discussed. The stability of nominally stoichiometric SrCeO3 was examined in an atmosphere of high water vapour partial pressure (pH2O) for 2 hours, degrading to Sr(OH)2.H2O and CeO2 for pH2O ≥ 3.6atm.[ES] La fase SrCeO3 da origen a una importante familia de perovskitas conductoras protónicas con potenciales aplicaciones tecnológicas. En este trabajo se estudia la formación de la fase y la estructura de SrCeO3 con la relación Sr:Ce no estequiométrica para la serie Sr1±xCeO3±δ (0.98 ≤ x ≤ 1.04). Los análisis por microsonda (EPMA) y difracción de rayos X (DRX) indican que en las muestras sinterizadas a 1350°C, la fase principal es rica en estroncio para todo valor de x. Se discute la posible ubicación del exceso de SrO tanto en la región intergranular como en el propio grano. También se examina la estabilidad de la composición con estequiometría nominal SrCeO3 en una atmosfera con una alta presión de vapor de agua (pH2O), observándose que la degradación a Sr(OH)2.H2O y CeO2 ocurre a pH2O ≥ 3.6atm (expuesto durante 2 horas).One of the authors (GCM) is sponsored by the EU RTN programme High Temperature Proton Conductors (HiTP) “Investigation of high temperature solid proton conductors of relevance to fuel processing and energy conversion applications”.Peer reviewe

The Science Performance of JWST as Characterized in Commissioning

This paper characterizes the actual science performance of the James Webb Space Telescope (JWST), as determined from the six month commissioning period. We summarize the performance of the spacecraft, telescope, science instruments, and ground system, with an emphasis on differences from pre-launch expectations. Commissioning has made clear that JWST is fully capable of achieving the discoveries for which it was built. Moreover, almost across the board, the science performance of JWST is better than expected; in most cases, JWST will go deeper faster than expected. The telescope and instrument suite have demonstrated the sensitivity, stability, image quality, and spectral range that are necessary to transform our understanding of the cosmos through observations spanning from near-earth asteroids to the most distant galaxies.Comment: 5th version as accepted to PASP; 31 pages, 18 figures; https://iopscience.iop.org/article/10.1088/1538-3873/acb29

The James Webb Space Telescope Mission

Twenty-six years ago a small committee report, building on earlier studies, expounded a compelling and poetic vision for the future of astronomy, calling for an infrared-optimized space telescope with an aperture of at least $4m$. With the support of their governments in the US, Europe, and Canada, 20,000 people realized that vision as the $6.5m$ James Webb Space Telescope. A generation of astronomers will celebrate their accomplishments for the life of the mission, potentially as long as 20 years, and beyond. This report and the scientific discoveries that follow are extended thank-you notes to the 20,000 team members. The telescope is working perfectly, with much better image quality than expected. In this and accompanying papers, we give a brief history, describe the observatory, outline its objectives and current observing program, and discuss the inventions and people who made it possible. We cite detailed reports on the design and the measured performance on orbit.Comment: Accepted by PASP for the special issue on The James Webb Space Telescope Overview, 29 pages, 4 figure

Crystallization Kinetics of LaF3 Nanocrystals in an Oxyfluoride Glass

Nanocrystallization of LaF3 in a glass of composition 55SiO2– 20Al2O3–15Na2O–10LaF3 (mol%) has been achieved by heat treatment above the glass transition temperature. A maximum crystal size of 14 nm has been attained, with the crystalline fraction and crystal size dependent on the time and temperature of thermal treatment. The effect of lanthanum fluoride crystallization is noticeable from the microstructural and compositional changes in the glass matrix, which have been studied using several techniques, including viscosity, dilatometry, X-ray diffraction, and quantitative Rietveld refinement, transmission electron microscopy, and differential scanning calorimetry. The crystallization mechanism is shown to occur via regions of La- and Si-phase separation in the glass, from which the fluoride crystals develop during heat treatment. The interface between the glass matrix and the crystals in the demixed ranges is enriched in network formers, mainly SiO2, creating a viscous barrier, which inhibits further crystal growth and limits the crystal size to the nanometric range.Peer reviewe

Estudio de los efectos producidos por la introducción de Br en el conductor protónico BaCe0.8Y0.2O3-δ

Resumen del trabajo presentado a la I Jornada de jóvenes investigadores de cerámica y vidrio en el ICMA, celebrada el 20 de marzo de 2018 en la Universidad de Zaragoza.Los ceratos de bario son ampliamente conocidos por su elevada conductividad protónica pero presentan baja estabilidad frente a gases ácidos como el CO2. Por ello, se pretende mejorar la estabilidad introduciendo halógenos como el bromo para reducir la electronegatividad de la estructura y disminuir su basicidad.Peer Reviewe

Electrical properties of proton-conducting BaCe0.8Y0.2O3-δ and the effects of bromine addition

[EN] A detailed analysis of the electrical properties of the proton-conducting BaCeYO phase (BCY20) and the effects of Br doping has been undertaken. Members of the system of nominal stoichiometry BaCeYOBr (x = 0, 0.05, 0.1 and 0.2) were synthesised by a citrate-nitrate process and high-temperature annealing up to 1500 °C. X-ray diffraction revealed the formation of single-phase material with a monoclinically distorted perovskite structure (space group I2/m) with greater distortion for Br-synthesised phase. Significant contrasts in electrical behaviour and stability between BCY20 and Br-synthesised series members were also found. However, chemical analysis by total X-ray fluorescence spectroscopy indicated that only trace amounts of Br remain after synthesis, indicating that Br addition influences stoichiometry but not directly the physicochemical properties. Higher conductivity is observed for the Br-synthesised compositions in wet and dry oxidising conditions in the temperature range 300–900 °C, reaching a value of 5.8 S m at 800 °C for the x = 0.2 member in wet O (pHO ∼ 0.022 atm). Determination of partial-conductivity components indicated that, in humid conditions and low temperature, the conductivity of 20% Br-synthesised material is principally protonic (t = 0.91 at 600 °C and pO = 0.2 atm) and superior to that of BCY20. Mixed oxide-ionic-electronic conductivity dominates at high pO (1 atm) and oxide-ionic conductivity at low pO (∼10 atm) in the temperature range 600–900 °C for both BCY and Br-doped samples. Stability was found to be poorer in CO for the Br-synthesised phases as determined by thermogravimetry and prolonged conductivity measurements.We thank MINECO in Spain forfinancial support (project ENE2015-66183-R and predoctoral grant BES-2016-077023). Wewould also like to thank Dr Ramón Fernández Ruiz of the Autonomous University of Madrid for useful discussions concerning TXRF measurements

Temperature dependence of partial conductivities of the BaZr0.7Ce0.2Y0.1O3-δ proton conductor

[EN] Partial conductivities are presented for BaZrCeYO, an important proton conductor for protonic-ceramic fuel cells and membrane reactors. Atmospheric dependencies of impedance performed in humidified and dry O, air, N and H(10%)/N(90%) in the temperature range 300–900 °C, supported by the modified emf method, confirm significant electron-hole and protonic contributions to transport. For very reducing and wet atmospheres, the conductivity is predominantly ionic, with a higher participation of protons with decreasing temperature and increasing water-vapour partial pressure (pHO). From moderately reducing conditions of wet N to wet O, however, the conductivity is considerably influenced by electron holes as revealed by a significant dependence of total conductivity on oxygen partial pressure (pO). With higher pHO, proton transport increases, with a concomitant decrease of holes and oxygen vacancies. However, the effect of pHO is also influenced by temperature, with a greater protonic contribution at both lower temperature and pO. Values of proton transport number t ≈ 0.63 and electronic transport number t ≈ 0.37 are obtained at 600 °C for pHO = 0.022 atm and pO = 0.2 atm, whereas t ≈ 0.95 and t ≈ 0.05 for pO = 10 atm. A hydration enthalpy of −109 kJ mol is obtained in the range 600–900 °C.We thank the “Ministerio de Economía, Industria y Competitividad” (MINECO) in Spain for financial assistance (ENE2015-66183-R)

Methodology for the study of mixed transport properties of a Zn-doped SrZr0.9Y0.1O3d electrolyte under reducing conditions

[EN] The mixed ionic-electronic transport properties of the protonic ceramic electrolyser material SrZr0.9Y0.1O3-, with the addition of 4mol% ZnO as sintering additive, are analysed under reducing conditions. The study is performed by means of an active-load modification of the classical electromotive-force method to account for the non-negligible effect of the electrodes on the obtained electrical-transport numbers. The methodology is developed in detail in order to link the electrochemical criteria to simulated equivalent circuits. The observed electromotive force of the system is considerably affected by the introduction of the polarisation resistance of the electrodes in the corresponding analysis, resulting in a high deviation between the present results and those obtained by a classical analysis without attending to electrode effects. Under wet reducing conditions (pH2 0.05 atm, pH2O 3·103102 atm), the oxide-ionic transport number is negligible in the range of 600900 ºC, whereas pure protonic conductivity is observed for temperatures ≤ 700 ºC and pH2O ≥ 5.6x103 atm. For higher temperatures and/or lower pH2O, mixed protonic-electronic conduction is exhibited. The electronic contribution under reducing conditions is consistent with n-type electronic behaviour.The authors acknowledge the financial support of MINECO (Plan Nacional, ENE2012-30929) and CSIC (i-link0743). We are grateful to Dr. M.J. Pascual (ICV, CSIC) for supplying the glass-ceramic seal

Transport-number determination of a protonic ceramic electrolyte membrane via electrode-polarisation correction with the Gorelov method

[EN] Analysis of transport numbers is critical for assessing the suitability of an ion-conducting material for a given electrochemical application and the conditions for its employment. In this work, the proton, oxide-ion and electron transport numbers of the candidate protonic ceramic electrolyser and fuel cell material SrZr0.9Y0.1O3-δ (with the addition of 4 mol% ZnO as sintering aid) are measured in wet and dry oxidising atmospheres in the temperature range 700-850 C. The determination of proton transport numbers is analysed in detail, encompassing the suitability of equivalent circuits in different conditions and the inclusion of an external parallel resistance for the correction of electrode-polarisation effects (Gorelov method). It is confirmed that transport numbers are highly inaccurate if no polarisation correction is applied. In dry oxidising conditions oxide-ion transport numbers, to, lie in the range 0.63-0.78. The conductivity in wet oxidising conditions is dominated by protons and an electronic component, with the proton transport number increasing from 0.79 to 0.88 with increasing pH2O in the range 1.1 × 10-3 ≤ pH2O ≤ 1.27 × 10-2 atm at 700 C. © 2013 Elsevier B.V. All rights reserved.We thank the“Ministerio de Economía y Competitividad”(MINECO) (Plan Nacional, ENE2012-30929) forfinancial assistance.D. Pérez-Coll also acknowledges the support of a“Ramón y Cajal”contract (MINECO, CSIC). We are grateful to Dr M.J. Pascual (ICV,CSIC) for supplying the sealing glassecerami