Pressure induced stability enhancement of cubic nanostructured CeO2

Ceria (CeO2)-based materials are widely used in applications such as catalysis, fuel cells and oxygen sensors. Its cubic fluorite structure with a cell parameter similar to that of silicon makes it a candidate for implementation in electronic devices. This structure is stable in a wide temperature and pressure range, with a reported structural phase transition to an orthorhombic phase. In this work, we study the structure of CeO2 under hydrostatic pressures up to 110 GPa simultaneously for the nanometer-and micrometer-sized powders as well as for a single crystal, using He as the pressure-transmitting medium. The first-order transition is clearly present for the micrometer-sized and single-crystal samples, while, for the nanometer grain size powder, it is suppressed up to at least 110 GPa. We show that the stacking fault density increases by two orders of magnitude in the studied pressure range and could act as an internal constraint, avoiding the nucleation of the high-pressure phase.Fil: Paulin, Mariano Andrés. Comisión Nacional de Energía Atómica. Gerencia del Área de Energía Nuclear. Instituto Balseiro. Archivo Histórico del Centro Atómico Bariloche e Instituto Balseiro | Universidad Nacional de Cuyo. Instituto Balseiro. Archivo Histórico del Centro Atómico Bariloche e Instituto Balseiro; ArgentinaFil: Garbarino, Gaston. European Synchrotron Radiation; FranciaFil: Leyva, Ana Gabriela. Comisión Nacional de Energía Atómica. Centro Atómico Constituyentes; Argentina. Universidad Nacional de San Martín. Escuela de Ciencia y Tecnología; ArgentinaFil: Mezouar, Mohamed. European Synchrotron Radiation; FranciaFil: Sacanell, Joaquin Gonzalo. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Unidad Ejecutora Instituto de Nanociencia y Nanotecnología. Unidad Ejecutora Instituto de Nanociencia y Nanotecnología - Nodo Constituyentes | Comisión Nacional de Energía Atómica. Unidad Ejecutora Instituto de Nanociencia y Nanotecnología. Unidad Ejecutora Instituto de Nanociencia y Nanotecnología - Nodo Constituyentes; Argentin

Structural and magnetic properties of oxides for spintronics

La espintrónica y su reciente desarrollo han fomentado el estudio de materiales en losque el magnetismo se encuentra asociado a sus propiedades eléctricas. El óxido de cerio (CeO2) es uno de los materiales más prometedores en este campo y sus propiedades hansido profundamente exploradas debido a su aplicación como catalizador y como materialpara celdas de combustible. En su estado original el CeO2 es diamagnético, pero dichoestado puede verse modificado al ser dopado con iones magnéticos o al incrementar lacantidad de defectos en su estructura. En esta Tesis presentaremos un estudio experimental y teórico de las propiedades estructurales,electrónicas y magnéticas de nanopartículas de óxido de cerio puro y dopadocon cobalto. El objeto principal del trabajo es entender el origen del magnetismo que se haobservado en los distintos sistemas de óxido de cerio. Para cumplir este objetivo sintetizamosmuestras puras y dopadas con cobalto al 0,5, 2, 4, 6, 7, 8, 10, 12 y 15% y realizamosmúltiples tratamientos térmicos en atmósfera reductora para inducir la formación de vacanciasde oxígeno. Para caracterizar las muestras recurrimos a una diversidad de técnicas:difracción de Rayos X, espectroscopia de fotoelectrones emitidos por Rayos X (XPS), resonanciaparamagnética electrónica (EPR), absorción de Rayos X (XANES), espectrometríaretrodispersiva de Rutherford (RBS), emisión de Rayos X inducida por partículas (PIXE)y magnetización DC. El estudio de las propiedades estructurales fue realizado mediante difracción de Rayos X en difractómetros convencionales y utilizando radiación sincrotrón. Esto último nospermitió detectar de cobalto segregado en las muestras dopadas que resultaban indetectablesmediante experimentos convencionales. Por otro lado, estudiamos la estabilidad dela estructura del óxido de cerio sometiéndolo a elevadas presiones hidrostáticas y logramosmediante la nanoestructuración aumentar el rango de estabilidad de la misma hasta 110GPa. Las mediciones de XPS indicaron un aumento significativo en la concentraciónde Ce3+ al realizar tratamientos térmicos reductores. Sin embargo, los experimentos de XANES y RBS arrojaron que la concentración del mismo estaba por debajo del límitede detección. Dado que XPS es una técnica de superfcie, esto estaría indicando que lareducción de las muestras en su interior es despreciable. En lo referente a las propiedades magnéticas, las muestras de óxido de cerio puro presentaronun comportamiento diamagnético previo al tratamiento térmico a baja presióny un comportamiento paramagnético luego de los mismos. En cambio, las muestras dopadascon cobalto resultaron paramagnéticas en todo el rango de temperatura estudiado. Lacaracterización de las muestras dopadas expuestas a tratamientos térmicos a baja presiónreveló un comportamiento similar al de un ferromagneto pero asociado a procesos de relajación. Los resultados indican que este fenómeno proviene de peque~nos clusters de cobaltometálico superparamagnéticos que realizan un aporte considerable a la se~nal magnética. Esta observación fue complementada con cálculos de primeros principios que nos permitieronobtener una comprensión microscópica de los resultados experimentales. Observamosque para el caso del óxido de cerio dopado, los cobaltos que se introducen sustitutivamentepresentan una tendencia a aglomerarse, lo cual podría dar origen a una nucleación y a unaposterior segregación. En vista de los resultados obtenidos decidimos extender el estudio en atmósfera reductora,lo cual nos permitiría obtener porcentajes considerables de Ce3+ y de esta manerainducir un comportamiento magnético. Realizamos mediciones de difracción, absorción de Rayos X y magnetización DC en flujo de hidrógeno y en vacío. Las mediciones en condicionesreductoras arrojaron resultados que permitieron comprender lo medido en condicionesnormales. Mediante la técnica de XANES determinamos que en condiciones reductoras sealcanzan elevados grados de reducción, observándose un porcentaje de Ce3+ cercano al 19% en vacío y un 26% en flujo de H2. Sin embargo, al bajar la temperatura y exponer lamuestra al aire, ésta se oxida dejando un nivel residual considerablemente bajo de Ce3+. En cuanto a las mediciones de magnetización, se realizaron tanto en flujo de hidrógenocomo en vacío, observando en ambos casos un cambio en el comportamiento magnético alaumentar la temperatura. Este cambio de diamagnetismo a paramagnetismo se conservó entodo el rango de temperatura (800-50K) y, dado que se llegaron a grados altos de reducción (superiores al 10% de Ce3+), permite descartar la hipótesis de ferromagnetismo en ceriapura presentada en la literatura .Spintronics and its recent development has driven the study of materials in whichmagnetism is linked to its electrical properties. Cerium oxide (CeO2) is one of the mostpromising materials in this field and its properties have been deeply explored due to itsapplication as a catalyst and fuel cell material. CeO2 is diamagnetic in its original statebut this can be modified by doping with magnetic ions or by increasing the number ofdefects in its structure. In this thesis we present an experimental and theoretical study of the structural, electronicand magnetic properties of cerium oxide nanoparticles, both pure and doped withcobalt ions. The main objective of this work is to understand the origin of the observedmagnetism in the different cerium oxide systems. To achieve this goal we synthesized pureand doped samples with 0,5, 2, 4, 6, 7, 8, 10, 12 and 15% cobalt concentration andperformed multiple thermal treatments in a reducing atmosphere to induce the formationof Oxygen vacancies. To characterize the samples, we used a variety of techniques: Xraydiffraction, X-ray photoelectron spectroscopy (XPS), electron paramagnetic resonance (EPR), X-ray absorption (XANES), Rutherford back scattering (RBS), particle induced X-ray emission (PIXE) and DC magnetization. The study of the structural properties was performed using X-ray diffraction in a conventionaldiffractometer and also by synchrotron radiation. This latter technique allowedus to detect segregated cobalt in the doped samples which were undetectable by conventionalexperiments. We also studied the stability of the cerium oxide structure underhigh hydrostatic pressures and , through nanostructuring, we obtained an increase in thestability range up to 110GPa. XPS measurements point towards a significant increase in Ce3+ concentration upon reducing thermal treatments. However, the XANES and RBSexperiments showed that this concentration was below the detection limit. Since XPS is asurface technique, this result would indicate that the reduction in the interior of the sample is negligible. Regarding the magnetic properties, pure cerium oxide samples showed a diamagneticbehavior prior to the heat treatment at low pressure and a paramagnetic response after thosetreatments The doped samples showed a paramagnetic behavior throughout the studiedtemperature range. The characterization of the doped samples exposed to heat treatmentsat low pressure revealed a behavior similar to that of a ferromagnet but associated torelaxation processes. Our results indicate that this phenomenon might arise from smallclusters of metallic cobalt which make a considerable contribution to the magnetic signal. This observation was complemented performing first principles calculations wich providedus a microscopic understanding of the experimental results. We observed that in the caseof doped cerium oxide, the substitutively introduced cobalt ions present a tendency toagglomerate, which in turn, could give rise to nucleation and posterior segregation. In the light of the obtained results, we decided to extend the study in reducing atmosphere,in order to increase the Ce3+ content and thus to induce the desired magneticproperties. We carried out measurements using X-ray Diffraction, X-ray Absorption and DC magnetization techniques in both hydrogen ux and vacuum. Experiments under reducingconditions allowed us an understanding of the measurements under normal conditions. By means of the XANES technique, we determined that in reducing conditions high levelsof reduction are attained, with a percentage of Ce3+ being close to 20% in vacuum and 30% in H2 ux. However, by decreasing the temperature and exposing the sample to air, itis oxidized leaving a substantially low residual level of Ce3+ ions. Regarding the magnetizationmeasurements, wich were performed both in hydrogen ux and vacuum, we observe inboth cases a change in the magnetic behavior as the temperature is increased. This changefrom diamagnetism to paramagnetism was preserved throughout the temperature range (800-50K) and given that we obtained high degrees of reduction (above 10% of Ce3+), ourresults allow us to rule out the pure cerium oxide ferromagnetism hypothesis proposed inthe literature.Fil: Paulin, Mariano Andrés. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales; Argentina

Magnetic properties of cobalt doped ZrO2 nanoparticles: Evidence of Co segregation

We synthesized pure and Co-doped (6.25%–12.5% at.) ZrO2 nanopowders in order to study their magnetic properties. We analyzed magnetic behavior as a function of the amount of Co and the oxygenation, which was controlled by low pressure thermal treatments. As prepared pure and Co-doped samples are diamagnetic and paramagnetic respectively. Ferromagnetism can be induced by performing low pressure thermal treatments, which becomes stronger as the dwell time of the thermal treatment is increased. This behavior can be reversed, recovering the initial diamagnetic or paramagnetic behavior, by performing reoxidizing thermal treatments. Also, a cumulative increase can be observed in the saturation of the magnetization with the number of low pressure thermal treatments performed. We believe that this phenomenon indicates that cobalt segregation induced by the thermal treatments is the responsible for the magnetic properties of the ZrO2-Co system.Fil: González Pinto, Francisco. Comisión Nacional de Energía Atómica. Centro Atómico Constituyentes; Argentina. Comisión Nacional de Energía Atómica. Gerencia de Área Investigaciones y Aplicaciones No Nucleares. Gerencia Física (CAC). Departamento de Física de la Materia Condensada; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Patagonia Norte; ArgentinaFil: Paulin, Mariano Andrés. Comisión Nacional de Energía Atómica. Centro Atómico Bariloche; Argentina. Comisión Nacional de Energía Atómica. Unidad Ejecutora Instituto de Nanociencia y Nanotecnología. - Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Unidad Ejecutora Instituto de Nanociencia y Nanotecnología; ArgentinaFil: Leyva de Guglielmino, Ana Gabriela. Comisión Nacional de Energía Atómica. Centro Atómico Constituyentes; Argentina. Comisión Nacional de Energía Atómica. Gerencia de Área Investigaciones y Aplicaciones No Nucleares. Gerencia Física (CAC). Departamento de Física de la Materia Condensada; Argentina. Comisión Nacional de Energía Atómica. Unidad Ejecutora Instituto de Nanociencia y Nanotecnología. - Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Unidad Ejecutora Instituto de Nanociencia y Nanotecnología; Argentina. Universidad Nacional de San Martín. Escuela de Ciencia y Tecnología; ArgentinaFil: Sacanell, Joaquin Gonzalo. Comisión Nacional de Energía Atómica. Centro Atómico Constituyentes; Argentina. Comisión Nacional de Energía Atómica. Gerencia de Área Investigaciones y Aplicaciones No Nucleares. Gerencia Física (CAC). Departamento de Física de la Materia Condensada; Argentina. Comisión Nacional de Energía Atómica. Unidad Ejecutora Instituto de Nanociencia y Nanotecnología. - Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Unidad Ejecutora Instituto de Nanociencia y Nanotecnología; Argentin

ICONE – Towards a French HiCANS Neutron Source for materials science and industry

We present the ICONE project which proposes to build a HiCANS source in France. The aim of the ICONE project is to be able to provide the French neutron user community sufficient instrumental capacity to continue performing neutron scattering experiments for their research programs. The baseline goal is to offer performances equivalent to a medium power research reactor or spallation source (such as Orphée or ISIS). We consider that such a machine would fulfil the needs of at least two-thirds of the users which not require ultimate performances but simply beam-time to perform their experiments. We also describe the experimental work ongoing at Saclay around the various technologies necessary to build a HiCANS

Microwave-assisted hydrothermal nanoarchitectonics of polyethyleneimine-coated iron oxide nanoparticles

The aim of this work is to show how the magnetic properties of polyethyleneimine (PEI)-coated and naked magnetite (Fe3O4) nanoparticles are improved when the alternative in situ PEI microwave-assisted hydrothermal synthesis method is employed in contrast to the results obtained for nanoparticles synthesized from the same precursors by a conventional co-precipitation method. As far as we know, this is the first report on “in situ” PEI-coated Fe3O4 nanoparticles obtained by microwave-assisted hydrothermal synthesis. For this purpose, uncoated and PEI-coated Fe3O4 nanoparticles were synthesized by the co-precipitation method (CCPM) using ammoniac as precipitating agent after heating at 80 °C and by a microwave-assisted hydrothermal method (MWAM) using urea at 140 °C during 30 min. The obtained powders so were characterized by X-ray diffraction, scanning and transmission electron microscopy, thermogravimetric analysis, Fourier transform infrared spectroscopy, Mössbauer spectroscopy and DC magnetization studies. It was seen that the magnetic properties of the nanoparticles strongly depend on the synthesis route and that they also vary if particles are coated by the nonmagnetic polymer (PEI). The morphology, size, surface and anisotropy effects play an important role in the magnetic behavior. In situ PEI-coated nanoparticles obtained by MWAM, with larger average size and better crystallinity, exhibited a higher coercive field and saturation magnetization values than PEI-coated nanoparticles synthesized by CCPM; the latter revealed a quasi-complete superparamagnetic behavior at room temperature. The synthesis by MWAM is a faster, simple and low-cost way to obtain in situ PEI-coated Fe3O4 nanoparticles with reasonable saturation magnetization, coercivity and suitable average particle size values to be tested for biomedical applications.Fil: Albornoz, Cecilia Andrea. Comisión Nacional de Energía Atómica. Gerencia de Área Investigaciones y Aplicaciones No Nucleares. Gerencia Física (CAC). Departamento de Física de la Materia Condensada; Argentina. Comisión Nacional de Energía Atómica. Unidad Ejecutora Instituto de Nanociencia y Nanotecnología. - Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Unidad Ejecutora Instituto de Nanociencia y Nanotecnología; ArgentinaFil: Paulin, Mariano Andrés. Comision Nacional de Energia Atomica. Gerencia D/area Invest y Aplicaciones No Nucleares. Departamento Haces de Neutrones del Ra10 - Cab.; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; ArgentinaFil: Cristobal, Adrian Alberto. Comisión Nacional de Energía Atómica. Centro Atómico Bariloche; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Mar del Plata. Instituto de Investigaciones en Ciencia y Tecnología de Materiales. Universidad Nacional de Mar del Plata. Facultad de Ingeniería. Instituto de Investigaciones en Ciencia y Tecnología de Materiales; ArgentinaFil: Vega, Daniel Roberto. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Mar del Plata. Instituto de Investigaciones en Ciencia y Tecnología de Materiales. Universidad Nacional de Mar del Plata. Facultad de Ingeniería. Instituto de Investigaciones en Ciencia y Tecnología de Materiales; ArgentinaFil: Leyva de Guglielmino, Ana Gabriela. Comisión Nacional de Energía Atómica. Gerencia de Área Investigaciones y Aplicaciones No Nucleares. Gerencia Física (CAC). Departamento de Física de la Materia Condensada; ArgentinaFil: Ramos, Cinthia Paula. Comisión Nacional de Energía Atómica. Gerencia de Área Investigaciones y Aplicaciones No Nucleares. Gerencia Física (CAC). Departamento de Física de la Materia Condensada; Argentina. Universidad Nacional de San Martín. Escuela de Ciencia y Tecnología; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentin

The HERMES reflectometer at the JULIC Neutron Platform

HERMES is a time-of-flight reflectometer that operated at the Orphée reactor until 2019. In 2022, HERMES was installed at the JULIC (Jülich Light Ion Cyclotron) Neutron Platform as part of a collaboration between the Laboratoire Léon Brillouin and the Jülich Centre for Neutron Science. The main goal of the current setup is to probe the viability of neutron instrumentation at a High Current Compact Accelerator-driven Neutron Source (HiCANS). As the flux at the JULIC neutron platform is several orders of magnitude lower than the original Orphée flux or the expected flux for a HiCANS, our current objective is to perform reflectivity experiments with supermirrors as a proof of concept. Nevertheless, Monte-Carlo simulations showed that the HERMES instrument’s performance at a HiCANS such as HBS or ICONE could match that of reflectometry instruments operating at research reactors or spallation sources. An experiment with a supermirror carried out in December 2022 allowed us to preliminary prove the feasibility of this kind of experiments at an accelerator-driven neutron source

Development of neutron reflectometry at a HiCANS: the HERMES instrument at the JULIC Neutron Platform

High current Compact Accelerator-driven Neutron Sources (HiCANS) have risen as a possible answer to the drop in neutron availability in recent years due to the closure of various research reactors in Europe. Within this new trend, the Laboratoire Léon Brillouin (LLB) is currently evaluating the performance of neutron techniques around this novel type of source. HERMES [1] is a time-of-flight horizontal reflectometer that was operated by the LLB at the Orphée reactor [2] until 2019 and was mainly employed for soft-matter studies. Through a collaboration with the Jülich Centre for Neutron Science, HERMES was installed in 2022 at the JULIC neutron platform at Forschungszentrum Jülich. This platform is able to deliver neutron pulses in the 100 μs-2 ms range and is very well suited to evaluate the feasibility of reflectivity experiments at a HiCANS. Since its installation and first tests in 2022, several improvements have been planned and implemented at HERMES in order to exploit its maximum performance. Our current goal is to perform reflectivity experiments with supermirrors as a proof of concept, as the flux at the JULIC neutron platform is several orders of magnitude lower than the original Orphée flux or the one expected for a HiCANS. Nevertheless, Monte-Carlo simulations showed that an instrument as HERMES operating at a HiCANS could match the performance of similar instruments at medium power research reactors. This work is part of the collaboration within ELENA and LENS on the development of HiCANS. It has been funded by the "CANS Inflexion" program at the CEA and the "IPHI-Neutron" SESAME project of the Île de France region.[1] F. Cousin, F. Ott, F. Gibert, A. Menelle, Eur. Phys. J. Plus 126, 109 (2011)[2] B. Farnoux, D. Cribier, Phys. B+C 120, 31-36 (1983

JULIC Neutron Platform, a testbed for HBS

The High-Brilliance neutron Source (HBS) project [1] develops a High-Current Accelerator-driven Neutron Source (HiCANS) with a pulsed proton beam, a peak current of 100 mA and an average power at the target of 100 kW. The concept of such a HiCANS was published some years ago [2] indicating the feasibility of such a facility with all of its components: high-current accelerator, target station with integrated moderator-reflector assemblies and neutron instruments. All components require engineering development and testing. The JULIC Neutron Platform was thus developed as a testbed for all components and the investigation of their interplay.The JULIC Neutron Platform uses a cyclotron providing a tunable pulsed proton beam with a low current but a variable frequency and pulse length to a spacious experimental area. A target station shielding is placed in its center with an empty inner core of 1 m3, able to accommodate different moderator-reflector assemblies as well as cryogenic moderators. The target station uses a tantalum target for the conversion of protons to neutrons and has eight spacious flexible ducts where moderator plugs for neutron extraction or blind plugs are placed.First beam on target was achieved in December 2022 with three instruments in operation: reflectometer, diffractometer and detector test stand. Further beamtime in 2023 is planned in order to investigate different cryogenic moderators, to estimate the performance of such a HiCANS and to perform further experiments.At UCANS, we will present the JULIC Neutron Platform, the experiments performed and the possibilities it offers.[1] P. Zakalek, et al, J. Phys.: Conf. Ser., 1401, 012010 (2020)[2] T. Brückel, et al. Conceptual Design Report Jülich High Brilliance Neutron Source (HBS), Forschungszentrum Jülich GmbH Zentralbibliothek, Verlag Jülich (2020

The High Brilliance neutron Source (HBS): A project for a next generation neutron research facility

The High Brilliance neutron Source (HBS) is a project for a next generation neutron research facility, based on new concepts and recent technological advancements. As elementary processes it uses neither fission nor spallation, but instead low energy nuclear reactions in a very compact Target-ModeratorReflector (TMR) assembly. Our facility design results in very efficient production of neutron beams with high brightness. Key features of HBS are: (i) very competitive instrument performance, (ii) comparatively low construction and operation costs, (iii) resilience, (iv) sustainability, (v) flexibility, (vi) accessibility and (vii) scalability. Here we present the basic layout of the facility, elaborate on the mentioned key features and report on the commissioning of a small test setup