Academic literature on the topic 'Perovskites and fluorites'

Metastable solid solutions of complex oxides with fluorite and perovskite structures are obtained by mechanosynthesis. Dense ceramics on the base of these metastable phases was obtained by thermal sintering of nanopowders due to kinetic stabilization. Different degrees of a chemical interaction (interdiffusion) are observed during sintering of "perovskite+fluorite" and "perovskite+perovskite" composites. It is shown, that optimization of the composition, mixing conditions of individual phases and their sintering, preparation of ceramic composites with mixed conductivity for use in catalytic membrane reactors is possible. Unusual behavior of complex perovskites and fluorites is discovered during sintering, enabling determination of an optimum sintering temperature and time for which a qualitative explanation is given. It is established that rearrangement of fine crystalline particles as a whole plays a key role in shrinkage.

AbstractOn the basis of extensive studies of synthetic perovskite-structured compounds it is possible to derive a hierarchy of hettotype structures which are derivatives of the arisotypic cubic perovskite structure (ABX3), exemplified by SrTiO3 (tausonite) or KMgF3 (parascandolaite) by: (1) tilting and distortion of the BX6 octahedra; (2) ordering of A- and B-site cations; (3) formation of A-, B- or X-site vacancies. This hierarchical scheme can be applied to some naturally-occurring oxides, fluorides,hydroxides, chlorides, arsenides, intermetallic compounds and silicates which adopt such derivative crystal structures. Application of this hierarchical scheme to naturally-occurring minerals results in the recognition of a perovskite supergroup which is divided into stoichiometric and non-stoichiometricperovskite groups, with both groups further divided into single ABX3 or double A2BB'X6 perovskites. Subgroups, and potential subgroups, of stoichiometric perovskites include: (1) silicate single perovskites of the bridgmanite subgroup;(2) oxide single perovskites of the perovskite subgroup (tausonite, perovskite, loparite, lueshite, isolueshite, lakargiite, megawite); (3) oxide single perovskites of the macedonite subgroup which exhibit second order Jahn-Teller distortions (macedonite, barioperovskite); (4) fluoride singleperovskites of the neighborite subgroup (neighborite, parascandolaite); (5) chloride single perovskites of the chlorocalcite subgroup; (6) B-site cation ordered double fluoride perovskites of the cryolite subgroup (cryolite, elpasolite, simmonsite); (7) B-site cation orderedoxide double perovskites of the vapnikite subgroup [vapnikite, (?) latrappite]. Non-stoichiometric perovskites include: (1) A-site vacant double hydroxides, or hydroxide perovskites, belonging to the söhngeite, schoenfliesite and stottite subgroups; (2) Anion-deficient perovskitesof the brownmillerite subgroup (srebrodolskite, shulamitite); (3) A-site vacant quadruple perovskites (skutterudite subgroup); (4) B-site vacant single perovskites of the oskarssonite subgroup [oskarssonite]; (5) B-site vacant inverse single perovskites of the coheniteand auricupride subgroups; (6) B-site vacant double perovskites of the diaboleite subgroup; (7) anion-deficient partly-inverse B-site quadruple perovskites of the hematophanite subgroup.

A mechanism of superfast mechanosynthesis reaction for oxide systems is proposed on the base of a dynamics study. The threshold effect and linear dependence of the chemical response on the effective temperature of the reaction zone are established. Major factors are determined: molecular mass of reagents, enthalpy and difference of reagents in Mohs?s hardness, which also influence the composition of the primary product. Primary acts are characterized by a superfast roller mechanism of mass transfer with the formation of a transient dynamic state (D)*. Secondary acts slowly approximate the composition of the product to the composition of the starting mixture by diffusion mass transfer in a deformation mixing regime with a contribution of a rotation (roller) mechanism. The list of structure types for complex oxides derived by mechanosynthesis includes perovskites, fluorites, pyrochlors, sheelites, and some other ones. Powders of crystal products display multilevel structurization. In all studied complex oxides strong disordering of the ?anti-glass? type was observed. The mechanism of sintering was studied in BaTiO3 powders of different origin and in metastable complex oxides derived by mechanosynthesis. The major contribution in shrinkage belongs to rearrangements of crystalline particles as a whole. Structure transformations accompany, as a rule, sintering of inhomogeneous powders derived by mechanosynthesis.

Metal halide perovskites have become a research highlight in the optoelectronic field due to their excellent properties. The perovskite light-emitting diodes (PeLEDs) have achieved great improvement in performance in recent years, and the construction of quasi-2D perovskites by incorporating large-size organic cations is an effective strategy for fabricating efficient PeLEDs. Here, we incorporate the fluorine meta-substituted phenethylammonium bromide (m-FPEABr) into CsPbBr3 to prepare quasi-2D perovskite films for efficient PeLEDs, and study the effect of fluorine substitution on regulating the crystallization kinetics and phase distribution of the quasi-2D perovskites. It is found that m-FPEABr allows the transformation of low-n phases to high-n phases during the annealing process, leading to the suppression of n = 1 phase and increasing higher-n phases with improved crystallinity. The rational phase distribution results in the formation of multiple quantum wells (MQWs) in the m-FPEABr based films. The carrier dynamics study reveals that the resultant MQWs enable rapid energy funneling from low-n phases to emission centers. As a result, the green PeLEDs achieve a peak external quantum efficiency of 16.66% at the luminance of 1279 cd m−2. Our study demonstrates that the fluorinated organic cations would provide a facile and effective approach to developing high-performance PeLEDs.

We computed the atomic shift sizes of the closest adjacent atoms adjoining the (001) surface F-center at ABO3 perovskites. They are significantly larger than the atomic shift sizes of the closest adjacent atoms adjoining the bulk F-center. In the ABO3 perovskite matrixes, the electron charge is significantly stronger confined in the interior of the bulk oxygen vacancy than in the interior of the (001) surface oxygen vacancy. The formation energy of the oxygen vacancy on the (001) surface is smaller than in the bulk. This microscopic energy distinction stimulates the oxygen vacancy segregation from the perovskite bulk to their (001) surfaces. The (001) surface F-center created defect level is nearer to the (001) surface conduction band (CB) bottom as the bulk F-center created defect level. On the contrary, the SrF2, BaF2 and CaF2 bulk and surface F-center charge is almost perfectly confined to the interior of the fluorine vacancy. The shift sizes of atoms adjoining the bulk and surface F-centers in SrF2, CaF2 and BaF2 matrixes are microscopic as compared to the case of ABO3 perovskites.

Organic–inorganic halide two-dimensional (2D) layered perovskites have been demonstrated to have better environmental stability than conventional three-dimensional perovskites. In this study, we investigate the fabrication of electron transport layer (ETL)-free Ruddlesden–Popper 2D perovskite solar cells (PSCs) by tuning the work function of a fluorine-doped tin oxide (FTO) electrode. With the deposition of polyethylenimine (PEIE) onto its surface, the work function of the FTO electrode could be raised from −4.72 to −4.08 eV, which is more suitable for electron extraction from the perovskite absorber. Using this technique, the ETL-free 2D PSCs exhibited an excellent power conversion efficiency (PCE) of 12.7% (on average), which is substantially higher than that of PSCs fabricated on a pristine FTO electrode (9.6%). Compared with the PSCs using TiO2, the ETL-free PSCs could be fabricated under a low processing temperature of 100 °C with excellent long-term stability. After 15 days, the FTO/PEIE-based ETL-free PSCs showed a PCE degradation of 16%, which is significantly lower than that of the TiO2-based case (29%). The best-performing PSC using a FTO/PEIE cathode showed a high PCE of 13.0%, with a small hysteresis degree of 2.3%.

Vanadium fluorides with novel crystal–chemical features and interesting physical properties can be prepared by solvothermal synthetic routes. The title compound, guanidinium hexafluoridovanadate(III), has a cubic structure (space group Pa\overline{3}), exhibiting isolated regular VF6 octahedral units, which are hydrogen bonded to protonated guanidinium moieties. Although the VF6 octahedral units are not linked directly together, there are structural similarities between this crystal structure and those of the wider family of perovskite materials, in particular, hybrid perovskites based on extended ligands such as cyanide. In this context, the octahedral tilt system of the present compound is of interest and demonstrates that unusual tilt systems can be mediated via `molecular' linkers which allow only supramolecular rather than covalent interactions.

Perovskite-type oxides have proven to be a versatile class of compounds with systematic study of their structure and various properties. Further structural variations and properties can be added by adding a second anionic species other than oxide, such as hydride, fluoride, nitride, or others. The different charge, covalency, size, and new modes of local coordination offer convenient ways to further control carrier doping, magnetism, conductivity, and even chemical reactivity. In this review we examine the recent work concerning various mixed-anion perovskites and conclude with potential new directions for the further development of these materials.

In recent years, great progress has been made in the field of energy harvesting to satisfy increasing needs for portable, sustainable, and renewable energy. Among piezoelectric materials, poly(vinylidene fluoride) (PVDF) and its copolymers are the most promising materials for piezoelectric nanogenerators (PENGs) due to their unique electroactivity, high flexibility, good machinability, and long–term stability. So far, PVDF–based PENGs have made remarkable progress. In this paper, the effects of the existence of various nanofillers, including organic–inorganic lead halide perovskites, inorganic lead halide perovskites, perovskite–type oxides, semiconductor piezoelectric materials, two–dimensional layered materials, and ions, in PVDF and its copolymer structure on their piezoelectric response and energy–harvesting properties are reviewed. This review will enable researchers to understand the piezoelectric mechanisms of the PVDF–based composite–film PENGs, so as to effectively convert environmental mechanical stimulus into electrical energy, and finally realize self–powered sensors or high–performance power sources for electronic devices.

This thesis primarily focuses on the systematic understanding of structure – reactivity relationships in two representative systems: bixbyite and related structures as well as indium doped CeO2. Topotactic reaction routes have gained significant attention over the past two decades due to their potential to access kinetically controlled metastable materials. This has contributed substantially to the understanding of solid state reaction pathways and provided first insights into mechanisms. Contrary to the widely used ex-situ methods, in – situ techniques including powder x-ray diffraction and thermogravimetric – differential thermal analysis have been employed extensively throughout this work in order to follow the reaction pathways in real time. Detailed analysis of the AVO3 (A = In, Sc) bixbyite reactivity under oxidative conditions has been carried out and a variety of novel metastable oxygen defect phases have been identified and characterized. The novel metastable materials have oxygen deficient fluorite structures and consequently are potential ion conductors. Structural aspects of the topotactic vs. reconstructive transformations are illustrated with this model system. The structure – reactivity study of AVO3 phases was extended to AVO3 perovskite family. Based on the research methodologies and results from AVO3 bixbyite reactivity studies a generalized mechanistic oxidation pathway has been established with a non-vanadium phase, ScTiO3 bixbyite. However, there is stark contrast in terms of structural stability and features beyond this stability limit during AVO3 and ScTiO3 bixbyite reaction pathways. A series of complex reaction sequences including phase separation and phase transitions were identified during the investigation of ScTiO3 reactivity. The two-step formation pathway for the fluorite – type oxide ion conductor Ce1-xInxO2-δ (0 ≤ x ≤ 0.3) is being reported. The formation of the BaCe1-xInxO3-δ perovskites and the subsequent CO2-capture reaction with the formation of Ce1-xInxO2-δ (0 ≤ x ≤ 0.3) has been investigated in detail. The two-step formation pathway is contrasted with the unsuccessful direct method. The stability and the extent of In – doping for the CeO2 fluorite phases that can be achieved through this CO2 – capture method are reported. The necessity and strategies for the selection of appropriate intermediate precursors for the preparation of doped CeO2 are also reported.

Los materiales conductores mixtos de electrones e iones (oxígeno o protones) son capaces de separar oxígeno o hidrógeno de los gases de combustión o de corrientes de reformado a alta temperatura. La selectividad de este proceso es del 100%. Estos materiales, óxidos sólidos densos, pueden usarse en la producción de electricidad a partir de combustibles fósiles, así como formar parte de los procesos que forman parte del sistema de captura y almacenamiento de CO2. Las membranas de transporte de oxígeno (MTO) se pueden utilizar en las plantas energéticas con procesos de oxicombustión, así como en reactores catalíticos de membrana (RCM), mientras que las membranas de transporte de hidrógeno (MTH) se aplican en procesos de precombustión. Además, estos materiales encuentran aplicación en componentes de sistemas energéticos, como electrodos o electrolitos de pilas de combustible de óxido sólido, de ambas clases iónicas y protónicas (SOFC y PC-SOFC). Los procesos mencionados implican condiciones de operación muy severas, como altas temperaturas y grandes gradientes de presión parcial de oxígeno (pO2), probablemente combinadas con la presencia de CO2 and SO2. Los materiales más que mayor rendimiento de separación presentan y más ampliamente investigados en este campo son inestables en estas condiciones. Por tanto, existe la necesidad de encontrar nuevos materiales inorgánicos estables que proporcionen alta conductividad electrónica e iónica. La presente tesis propone una búsqueda sistemática de nuevos conductores iónicos-electrónicos mixtos (MIEC, del inglés) con diferente estructura cristalina y/o diferente composición, variando la naturaleza de los elementos y la estequiometría del cristal. La investigación ha dado lugar a materiales capaces de transportar iones oxígeno, protones o cargas electrónicas y que son estables en las condiciones de operación. La caracterización de una amplia serie de cerias (CeO2) dopadas con lantánidos proporciona una comprensión general de las propiedades estructurales y de transporte, así como la relación entre ellas. Además, se estudia el efecto de la adición de cobalto a dicho sistema. Se ha completado el análisis con la optimización de las propiedades de trasporte a partir de la microestructura. Todo esto permite hacer una clasificación inicial de los materiales basada en el comportamiento de transporte principal y permite adecuar la estructura y las condiciones de operación para obtener las propiedades deseadas para cada aplicación. Algunos de los materiales extraídos de este estudio alcanzaron las expectativas. Las familias de materiales basadas en Ce1-x Tbx O2-¿ y Ce1-x Tbx O2-¿ +2 mol% Co proporcionan flujos de oxígeno bajos pero competitivos, ya que son estables en atmósferas con CO2. Además, la inclusión de estos materiales en membranas de dos fases aumenta el flujo de oxígeno. La combinación con una espinela libre de cobalto y de metales alcalinotérreos como es el Fe2 NiO4, ha dado lugar a un material prometedor en cuanto a flujo de oxígeno y estabilidad en CO2 y en SO2, que podría ser integrado en el proceso de oxicombustión. Por otra parte, se ha añadido metales como codopantes en el sistema Ce0.9-x Mx Gd0.1O1.95. Estos materiales, en combinación con la perovskita La1- x Srx MnO3 usada comúnmente como cátodo de SOFC, han sido capaces de disminuir la resistencia de polarización del cátodo. La mejora es consecuencia de la introducción de conductividad iónica por parte de la ceria. Las perovskitas dopadas basadas en CaTiO3 forman el segundo grupo de materiales investigados. La dificultad de obtener perovskitas estables y que presenten conducción mixta iónica y electrónica se ha hecho evidente. De entre los dopantes utilizados, el hierro y la combinación hierro-magnesio han sido los mejores candidatos. Ambos materiales presentan conductividad principalmente iónica a alta temperatura, mientras que a baja predomina la conductividad electrónica tipo p. CaTi0.73Fe0.18Mg0.09O3-¿ se ha mostrado como un material competente en la fabricación de membranas de oxígeno, que proporciona flujos adecuados a la par que estabilidad en CO2. Finalmente, la perovskita La0.87Sr0.13CrO3 (LSC) ha sido dopada con el objetivo de aumentar la conductividad mixta protónica electrónica. Este estudio ha llevado al desarrollo de una nueva generación de ánodos para PC-SOFC basadas en electrolitos de LWO. Las perovskitas dopadas con Ce en el sitio del La (LSCCe) y con Ni en el sitio del Cr (LSCN) son estables en condiciones de operación reductoras, así como en contacto con el electrolito. El uso de ambos materiales como ánodo disminuye la resistencia de polarización con respecto al LSC. El LSCCe está limitado por los procesos que ocurren a baja frecuencia (BF), relacionados con los procesos superficiales, y que son atenuados en el caso del LSCN debido a la formación de nanopartículas de Ni metálico en la superficie. La infiltración posterior con nanopartículas de Ni permite disminuir la resistencia a BF lo que sugiere que la reacción superficial de oxidación del H2 está siendo catalizada. La infiltración más concentrada en Ni (5Ni) elimina completamente la resistencia a BF en ambos ánodos, de forma que los procesos que ocurren a altas frecuencias son ahora limitantes. El ánodo constituido por LSCNi20+5Ni dio una resistencia de polarización de 0.26 ¿·cm 2 at 750 ºC en H2 húmedo.
Mixed ionic (oxygen ions or protons) and electronic conducting materials (MIEC) separate oxygen or hydrogen from flue gas or reforming streams at high temperature in a process 100% selective to the ion. These solid oxide materials may be used in the production of electricity from fossil fuels (coal or natural gas), taking part of the CO2 separation and storage system. Dense oxygen transport membranes (OTM) can be used in oxyfuel combustion plants or in catalytic membrane reactors (CMR), while hydrogen transport membranes (HTM) would be applied in precombustion plants. Furthermore, these materials may also be used in components for energy systems, as advanced electrodes or electrolytes for solid oxide fuel cells (SOFC) and proton conducting solid oxide fuel cells (PCSOFC) working at high and moderate temperature. The harsh working conditions stablished by the targeted processes include high temperatures and low O2 partial pressures (pO2), probably combined with CO2 and SO2 containing gases. The instability disadvantages presented by the most widely studied materials for these purposes make them impractical for application to gas separation. Thus, the need to discover new stable inorganic materials providing high electronic and ionic conductivity is still present. This thesis presents a systematic search for new mixed ionic-electronic conductors. It includes different crystalline structures and/or composition of the crystal lattice, varying the nature of the elements and the stoichiometry of the crystal. The research has yielded new materials capable to transport oxygen ions or protons and electronic carriers that are stable in the working condition to which they are submitted.
Balaguer Ramírez, M. (2013). New solid state oxygen and hydrogen conducting materials. Towards their applications as high temperature electrochemical devices and gas separation membranes [Tesis doctoral no publicada]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/31654
TESIS
Premiado

Fast oxide-ion and proton conductors are the subject of considerable research due to their technological applications in sensors, ceramic membranes and solid oxide fuel cells (SOFCs). This thesis describes the use of computer modelling techniques to study point defects, dopants and clustering effects in fluorite-and perovskitetype ion conductors with potential SOFC applications. Bi2O3 related phases are being developed with the objective of high oxide-ion conductivities at lower operating temperatures than 1000°C, as in current generation SOFC electrolytes. Doped Bi2O3 phases have shown promise as materials capable of accomplishing this goal. First, the Y-doped phase, Bi3YO6, has been investigated including the ordering of intrinsic vacancies. The defect and dopant characteristics of Bi3YO6 have been examined and show that a highly mobile oxygen sub-lattice exists in this material. A preliminary structural modelling study of a new Re-doped Bi2O3 phase was also undertaken. A comprehensive investigation of the proton-conducting perovskites BaZrO3, BaPrO3 and BaThO3 is then presented. Our results suggest that intrinsic atomic disorder in BaZrO3 and BaThO3 is unlikely, but reduction of Pr4+ in BaPrO3 is favourable. The water incorporation energy is found to be less exothermic for BaZrO3 than for BaPrO3 and BaThO3, but in all cases the results suggest that the proton concentration would decrease with increasing temperature, in accord with experimental data. The high binding energies for all the dopant-OH pair clusters in BaPrO3 and BaThO3 suggest strong proton trapping effects. Finally, a study of multiferroic BiFeO3 is presented, in which the defect, dopant and migration properties of this highly topical phase are investigated. The reduction process involving the formation of oxygen vacancies and Fe2+ is the most favourable redox process. In addition, the results suggest that oxide-ion migration is anisotropic within this system.

Topochemical strategies, techniques that allow one to effectively manipulate the structures of nonmolecular solids once a crystal lattice is established, are effective in the low temperature (< 500 °C) modification of solid state structures, allowing the preparation of nonmolecular compounds not accessible by standard synthetic routes. Some of the techniques, ion exchange, intercalation/deintercalation, have proven to be excellent synthetic methods for preserving specific frameworks. The combination of these techniques can allow one to create a multistep approach that can be used to design new compounds with interesting properties. As an expansion to the field of topotactic reactions, a multistep approach was developed towards the synthesis of the new compounds (A xM0.5Cly)LaNb2O7 (where A = Rb, Cs; M = Fe, Ni; x ≈ 1.5;y ≈ 1) at temperatures below 400oC. The first reaction step involved the ion exchange of the host materials (ALaNb2O7, A = Rb, Cs) to form the products M0.5LaNb2O7 (where M = Fe, Ni), a structure open to further chemistry. The next step involved reductive intercalation with Rb or Cs metal to form the air sensitive mixed-valence products with the nominal compositions, A1.5M0.5LaNb2O7. The last step involved the oxidative intercalation of chlorine using chlorine gas to obtain the final compounds. This multistep approach is a design to form mix-metal halide layers, specifically those with divalent cations, within layered perovskites, opening the doors to compounds that can have interesting properties. This reaction series was also applied to the tantalate layered oxides, leading to the formation of the new compound Ni 0.5LaTa2O7 through ion exchange. The further multistep topochemical manipulation of this new compound was not successful and was indicative of the difference in chemical behavior of the tantalates versus the niobates. We have also investigated the oxidative intercalation of halogens into a series of Ruddlesden-Popper (R-P) ruthenate oxides with the formula Ae n+1RunO3n+1 (Ae = Ca, Sr; n = 1, 2, 3) using several sources of fluorine, chlorine, and bromine. A new method was developed to intercalate chlorine into layered systems; this new approach avoids the use of chlorine gas which is highly toxic. The new phase Sr3Ru2O7Cl0.7 was synthesized by the new method and further topotactic manipulations were explored. The chemistry was not limited to the n = 2 phase but was also applied to the n = 3 phase, Sr4Ru3O10.

Übergangsmetallfluoridverbindungen sind sehr vielversprechende Kandidaten für die nächste Generation von Kathoden für Alkaliionenbatterien. Dennoch verhindern einige Nachteile dieser Materialklasse ihre Anwendung in Energiespeichermedien. Metallfluoride haben eine stark isolierende Wirkung, außerdem bewirken die Mechanismen beim Lade-/Entladevorgang, große Volumenänderungen und somit eine drastische Reorganisation des Materials, welche nur geringfügig umkehrbar ist. Um diese Nachteile zu reduzieren, werden in dieser Arbeit innovative Syntheserouten für die Umwandlung von Metallfluoridverbindungen sowie deren Anwendung in Alkaliionenbatterien vorgestellt. Im ersten Teil werden MFx Verbindungen (M = Co, Fe; x = 2 oder 3) untersucht. Diese Materialien zeigen eine hohe Ausgangskapazität aber nur bei sehr geringen C-Raten und zudem sehr geringe Zyklisierbarkeiten. Ex-situ-XRD und -TEM zeigen, dass die geringe Umkehrbarkeit der Prozesse hauptsächlich aus der Umwandlungsreaktion während des Be-/Entladens resultieren. Im zweiten Teil werden sowohl die Synthesen als auch die elektrochemischen Eigenschaften von Perowskiten aus Übergangsmetallfluoriden vorgestellt. NaFeF3 zeigt hierbei exzellente Leistungen und Reversibilitäten. Die Untersuchung der Mechansimen durch ex-situ und operando XRD während der Be- und Entladeprozesse hinsichtlich verschiedener Alkalisysteme zeigt, dass das kristalline Netzwerk über den Zyklus erhalten bleibt. Dies führt zur hohen Reversibilität und hohen Leistung selbst bei hohen C-Raten. Der Erhalt der Kristallstruktur wird durch elektrochemische Stabilisierung der kubischen Konformation von FeF3 ermöglicht, welche normalerweise erst bei hohen Temperaturen (400 °C) beobachtet wird und durch geringere Reorganisationen innerhalb des Kristallgerüsts erklärt werden kann. Ähnliche elektrochemische Eigenschaften können für KFeF3 und NH4FeF3 beobachtet werden, wobei erstmalig von Ammoniumionen als Ladungsträger in Alkaliionensystemen berichtet wird.
Metal fluoride compounds appear as very appealing candidates for the next generation of alkali-ion battery cathodes. However, many drawbacks prevent this family of compounds to be applicable to storage systems. Metal fluorides demonstrate a high insulating character, and the mechanisms involved during the discharge/charge processes atom engender large volume changes and a drastic reorganization of the material, which induces poor reversibility. In order to answer these problematics, the present thesis reports the elaboration of innovative synthesis routes for transition metal fluoride compounds and the application of these fluoride materials in alkali-ion battery systems. In a first part, MFx compounds (M = Co, Fe; x = 2 or 3) are studied. Those compounds exhibit high initial capacity but very poor cyclability and low C-rate capabilities. Ex-situ X-ray diffraction and transmission electron microscopy demonstrate that the low reversibility of the processes is mainly due to the conversion reaction occurring during their discharge/charge. In the second part, the syntheses of transition metal fluoride perovskites are reported, as well as their electrochemical properties. NaFeF3 demonstrates excellent performances and reversibility. The study of the mechanisms occurring during its charge/discharge processes towards different alkali systems by ex-situ and operando X-ray diffraction reveals that its crystalline framework is maintained along the cycles, resulting in high reversibility and excellent C-rate performance. This retention of the crystal framework is possible by an electrochemical stabilization of a cubic conformation of FeF3, which is usually only observable at high temperature (400 °C), and can be explained by lower reorganizations within the crystal framework. Similar electrochemical properties could be observed for KFeF3 and NH4FeF3, where ammonium ions are reported for the first time as a charge carrier in alkali-ion systems.

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Tese (Doutorado em Tecnologia Nuclear)
IPEN/T
Instituto de Pesquisas Energeticas e Nucleares - IPEN-CNEN/SP

The present Thesis is focused on the development of ceramic membranes for the production of O2, as well as their use in several industrial applications (e.g. power generation, chemical industry). Different materials such as perovskites (BSCF and LSCF), fluorites (CGO) and composites, different membrane architectures have been considered. Catalytic activation was considered for the optimization of permeation, and for improving the selectivity/yield of chemical reactions. In the chapter dedicated to BSCF, the influence of thickness and the use of porous supports in the permeation was studied. An improvement in the permeation was observed for the thinner membranes. With respect to the porous supports, it was found that they contribute with an additional resistance within the permeation process, reducing the potential improvement when reducing thickness. The conducted tests also allowed to study more in deep the different processes affecting oxygen membranes, as well as defining a permeation model for monolithic and asymmetric membranes. Aiming to improve the surface reactions involved in the oxygen permeation the use of catalytic layers was considered, by means the addition of porous BSCF backbones. The best results were obtained when coating both sides of membranes with catalytic layers. The concept of BSCF activated membranes was also considered for the production of C2H4 by means of the oxidative de-hydrogenation of C2H6, obtaining high C2H4 yields. BSCF membranes presenting tubular geometry were characterized for application such as production of O2 and production of C2H4 by means of oxidative coupling of CH4. LSCF was considered for conducting studies under CO2-containing atmospheres. For both systems it was conducted a complete permeation study with a focus on permeation performance under CO2 environments. Furthermore a study focused on the different substrates was carried out for determining the structure presenting the lower gas diffusion resistance. Despite very good results were obtained for both membrane types, even under CO2 conditions, freeze casted membranes reached higher oxygen fluxes, being optimized with the catalytic activation of membranes. Materials presenting fluorite structure stand out for their stability under reaction conditions or when exposed to CO2 environments. Nevertheless, delivered oxygen fluxes are typically low. Hence, a thin 40 micron-thick CGO-Co membrane activated with Pd nanoparticles was considered for conducting a study on O2 permeation performance, and its behaviour when exposed to CO2 and CH4-containing atmospheres. A good stability was demonstrated, as well as a significant improvement in oxygen permeation when exposed to CH4 environments. Thus, CGO membranes present promising properties for their application in oxyfuel and for the conduction of chemical reactions. Composite materials based on NFO-CTO was carried out. An evaluation of the CTO content and its relation with permeation was conducted, determining that a higher ionic phase ratio in the membrane results in a higher permeation. A composite consisting of 50NFO-50CTO was considered for performing a permeation study under harsh application conditions, with presence of SO2. Despite the significant loss in permeation, the composite material resulted to be stable after a long exposure to SO2. A broad study about the effect of CO2 and SO2 on the oxygen surface reactions was conducted by means of EIS measurements on 60NFO-40CTO electrodes. It was observed a significant effect of SO2 on the surface exchange reactions by promoting the deactivation of the O2 active sites, due to a SO2 adsorption on them. This effect was minimized by activating 60NFO-40CTO backbones with different catalysts, being characterized by EIS under CO2&SO2 conditions. This improvement was later confirmed when performing permeation tests. Permeation was improved notably by reducing membrane thickness, depositing composite membranes on LSCF porous substrates.
La presente tesis trata sobre el desarrollo de membranas cerámicas para la producción de O2, así como de su uso en distintas aplicaciones industriales (producción de energía, industria química). Se han considerado distintos tipos de materiales tales como perovskitas (BSCF y LSCF), fluoritas (CGO) y materiales composites, así como distintas arquitecturas de membrana. y activación catalítica para optimizar la permeación y la selectividad/rendimiento en reacciones químicas. Para el BSCF se estudió la influencia del espesor y el uso de soportes porosos en la permeación de O2, con una mejora para las membranas más finas, y también el papel de los soportes porosos, contribuyendo con una resistencia adicional en el proceso de permeación. El estudio permitió también conocer más en profundidad los procesos que afectan a los distintos tipos de membranas, y establecer un modelo de permeación para membranas. Se recurrió a la activación catalítica mediante la adición de capas porosas de BSCF, obteniendo así mejores resultados para las membranas con capas en ambos lados. El concepto de membranas de BSCF activadas superficialmente se consideró también para la producción de C2H4 a partir de la deshidrogenación oxidativa de etano (ODHE), obteniendo rendimientos de C2H4 muy elevados. Membranas de BSCF con geometría tubular fueron caracterizadas para aplicaciones de producción de O2 y C2H4 mediante acoplamiento oxidativo de metano (OCM). Se consideró al LSCF para su uso en aplicaciones con atmósferas conteniendo CO2. Se desarrollaron membranas soportadas en soportes porosos de LSCF mediante tape casting y freeze-casting, realizando completos estudios de permeación, además de estudiar el tipo de soporte poroso ofreciendo menos resistencia a la difusión de los gases. Pese que para ambos tipos de membranas se obtuvieron muy buenos flujos de oxígeno, incluso bajo condiciones de CO2, para el caso de membranas con soporte fabricado mediante freeze-casting se consiguieron mayores valores de permeación, optimizándolos incluso con la activación catalítica. Los materiales con estructura fluorita poseen alta estabilidad bajo condiciones de reacción (atmósferas reductoras) o cuando son expuestos a CO2 (aplicaciones de producción de energía). Sin embargo, los valores de permeación suelen ser muy bajos. Se consideró una membrana de CGO-Co de 40 micras de espesor activada con nanopartículas de Pd para llevar a cabo un estudio de sus propiedades para la producción de O2, su comportamiento en contacto con CO2 y con atmósferas conteniendo CH4. La buena estabilidad demostrada y la mejora sustancial de los flujos de O2 bajo ambientes reductores, hacen que este tipo de materiales posean propiedades prometedoras para aplicaciones de oxicombustión y reacciones químicas. Se realizó un estudio con materiales composites formados por NFO-CTO. Una evaluación del contenido en CTO y su relación con la permeación de O2, resultó en mayores valores para composiciones con mayor contenido en CTO. Un composite consistente en 50NFO-50CTO se consideró para la realización de tests bajo condiciones de oxicombustión, con presencia de SO2. Pese al notable descenso en los flujos de O2, el material resultó ser completamente estable tras una exposición continuada al SO2. Un amplio estudio del efecto del CO2 y del SO2 sobre las reacciones superficiales se realizó mediantes medidas de EIS en electrodos de 60NFO-40CTO, demostrando que el SO2 afecta significativamente a las reacciones superficiales mediante procesos de adsorción competitiva en los centros activos. Se minimizó el efecto del SO2 sobre las reacciones de intercambio superficial al activar las membranas con capas catalíticas porosas de 60NFO-40CTO con distintos catalizadores, confirmando posteriormente esta mejora en tests de permeación en las mismas condiciones. Así mismo, se optimizó notablemente la permeación de las membranas de 60NFO-40CTO reduciendo el espes
La present tesi tracta sobre el desenvolupament de membranes ceràmiques per a la producció d'O2, així com del seu ús en diverses aplicacions industrials (producció d'energia, indústria química). S'han considerat diversos materials tals com perovskites (BSCF i LSCF), fluorites (CGO) i materials composites, així com diferents arquitectures de membrana i l'activació catalítica per a millorar la permeació i la sel·lectivitat/rendiment de les reaccions químiques. Per al BSCF s'estudià la influència de l'espessor i l'ús de suports porosos en la permeació d'O2, amb una millora dels fluxos d'O2 per al cas de les membranes més fines, i també el paper dels suports porosos, els quals contribueixen afegint una resistència al procés de permeació. L'estudi també va permetre conèixer més en profunditat els processos que afecten als diferents tipus de membranes, i establir un model de permeació per a membranes. Es va recórrer a l'activació catalítica mitjançant l'adició de capes poroses de BSCF, obtenint així millors resultats per a les membranes activades a ambdós costats. El concepte de membranes de BSCF activades superficialment es va considerar també per a la producció d'etilè a mitjançant la deshidrogenació oxidativa d'età (ODHE), obtenint rendiments de C2H4 molt elevats. Membranes de BSCF amb geometria tubular van ser caracteritzades per a aplicacions de producció d'O2 i C2H4 mitjançant l'acoplament oxidatiu de metà (OCM). Es va considerar al LSCF per al seu ús en aplicacions amb atmosferes contenint CO2. Així doncs, es van desenvolupar membranes suportades sobre suports porosos de LSCF fabricats per tape càsting i freeze càsting. Es van realitzar estudis complets de permeació per a ambdós casos, a més d'estudiar el tipus de suport porós que ofereix una menor resistència a la difusió dels gasos. Malgrat que es van obtindré molts bons fluxos d'O2 per als dos tipus de membranes, inclús sota condicions amb CO2, per al cas de les membranes amb suport fabricat per freeze càsting es van aconseguir majors valors de permeació, sent inclús optimitzats amb l'activació catalítica. Els materials amb estructura fluorita destaquen per l'alta estabilitat sota condicions de reacció (atmosferes reductores) o quan són exposats a CO2 (aplicacions per a la producció d'energia). Malgrat això, els valors de permeació solen ser molt baixos. Es va considerar una membrana de CGO-Co de 40 micras d'espessor activada amb partícules de Pd per a realitzar un estudi sobre les seues propietats en quant a la producció d'O2, el seu comportament amb el contacte amb CO2 i atmosferes reductores contenint CH4. La bona estabilitat demostrada i una millora substancial dels fluxos d'O2 sota ambients reductors fan que aquest tipus de material presente propietats prometedores per a aplicacions d'oxicombustió i reaccions químiques. Es va realitzar un estudi sobre materials composites formats per NFO-CTO. Es va realitzar una avaluació del contingut en CTO i la relació amb la permeació, observant una millora de la permeació amb un major contingut de CTO. Un composite consistent en 50NFO-40CTO es va considerar per a la realització de tests de permeació en condicions d'oxicombustió amb presència de SO2. Malgrat el notable descens en els fluxos d'O2, el material resultà ser estable després d'una exposició continuada al SO2. Es mesurà l'efecte del CO2 i del SO2 sobre les reaccions superficials fent ús de la tècnica d'EIS en elèctrodes de 60NFO-40CTO. Demostrant que el SO2 afecta significativament a les reaccions superficials degut a una adsorció competitiva O2-SO2 als centres actius. Es minimitzà l'efecte del SO2 sobre les reaccions superficials al activar les membranes amb capes poroses de 60NFO-40CTO amb diferents catalitzadors. Aquestes capes van ser caracteritzades per EIS sota condicions de SO2, confirmant posteriorment la millora al realitzar tests de permeació. S'optimitzà notablement la perme
García Fayos, J. (2017). DEVELOPMENT OF CERAMIC MIEC MEMBRANES FOR OXYGEN SEPARATION: APPLICATION IN CATALYTIC INDUSTRIAL PROCESSES [Tesis doctoral no publicada]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/86189
TESIS
Premiado

La recherche de nouveaux pigments colores dans le domaine des composes de type nitrure a conduit a la synthese et la caracterisation de nouveaux oxynitrures dans les systemes m-ta(nb)-o-n (m : alcalinoterreux) et ln-w-o-n (ln : element de terre rare). De nouveaux oxydes de type perovskite ont ete decouverts dans le systeme sr-ta-o. Ils forment un domaine de solution solide entre les compositions sr(sr 0. 3 7ta 0. 6 3)o 2. 9 5@ 0. 0 5 et sr(sr 0. 5ta 0. 5)o 2. 7 5@ 0. 2 5. Les oxynitrures correspondants sont isotypes des oxydes precurseurs et presentent une gamme de couleurs s'etendant du jaune pale a l'orange lorsque l'enrichissement en azote croit. L'influence de la nature de l'alcalinoterreux ou du metal de transition sur la couleur a ete determinee grace a l'etude des systemes ca(sr,ba)-ta(nb)-o-n ; un nouvel oxynitrure sr 2nbo 3n a alors ete decouvert. Il est isotype de sr 2tao 3n dont la structure cristalline et electronique est reportee. Cet oxynitrure de type k 2nif 4 possede un reseau anionique partiellement ordonne, inedit. L'origine de sa couleur orange vif est un mecanisme de transition bande a bande (bv / bc). Le role determinant de l'azote dans la couleur est confirme par des calculs de structure de bande sur un ensemble de nitrures et oxynitrures : par rapport a l'oxyde, la presence des orbitales 2p de l'azote au sommet de la bande de valence retrecit la bande interdite et la couleur des oxynitrures evolue du blanc au jaune, a l'orange, au rouge puis au brun selon la valeur du gap. Dans les systemes ln-w-o-n, deux familles d'oxynitrures de structure apparentee a la fluorine ont ete decouvertes a partir des oxydes ln 1 4w 4o 3 3 (formulation de type fluorine a 4x 7. 3 3@ 0. 6 7, a : cations, x : anions) et ln 6wo 1 2 (a 4x 6. 8 5@ 1. 1 5). Ces oxynitrures forment des domaines de solution solide entre l'oxyde de depart et un oxynitrure de formulation limite a 4x 6@ 2. La structure des tungstates ln 6wo 1 2 (ln : y, ho) a ete precisee. On montre que les oxynitrures a 4x 6@ 2 correspondants possedent une structure tres proche de la fluorine. La gamme de couleur des oxynitrures s'etend du jaune pale au marron et kaki, respectivement, pour les series ln 1 4w 4o xn y et ln 6wo xn y, lorsque la teneur en azote croit. On trouve, pour des compositions intermediaires, des poudres de couleur jaune vif prometteuses dans la perspective d'une utilisation comme pigments colores.

碩士
國立臺灣科技大學
化學工程系
102
This study aims to investigate the recovery of fluoride (626.5 mg/L) from an industrial wastewater in forms of perovskite-like minerals. Effects of equilibrium pH (pHeq), molar ratio of Na to F (Na/F), molar ratio of Al to F (Al/F), and different pH reagents were examined. Fluoride was recovered as cryolite (Na3AlF6) and elpasolite (K2NaAlF6) when pH of wastewater adjusted by NaOH and KOH, respectively. Once NaOH was used as pH reagent, fluoride removal efficiency was higher than 94.5% at Na/F of 12/6, Al/F of 1/6 and pHeq range of 5.5 to 7.2, but it decreased to 78.0% as pHeq became higher than 7.6. Although characteristic peaks of NaAlF4‧0.83H2O and AlF2OH‧1.4H2O showed in crystal patterns, it is noted that the amount of byproducts was insignificant from wet chemical analysis. Therefore, Na3AlF6 was the main precipitate recovered from wastewater at pHeq range of 3.4 to 7.2. However, increasing Al/F from 1/6 to 2.5/6, did not enhance removal efficiency at pHeq 5.5. PHREEQC simulation is conducted to predict and compare precipitation and speciation of Na3AlF6 with experimental ones. On the other hand, similar fluoride removal efficiency (>94.7%) was obtained using KOH as pH reagent at Na/F of 3/6 and Al/F of 1/6 when pHeq ranged from 4.2 to 5.7. As Na/F reduced to 1/6, it was obtained the lower fluoride removal efficiency except for pHeq 5.5. Morphology of precipitates recovered from wastewater was significantly affected by Na/F, Al/F and pHeq. Size analysis and sedimentation test showed that both K2NaAlF6 and Na3AlF6 could be easily separated from water.

Finely dispersed (СeO2)1-x(Sm2O3)x (x = 0.02; 0.05; 0.10); La1-xSrxNiO3, La1-xSrxCoO3 and La1-xSrxFe0.7Ni0.3O3 (x = 0.30; 0.40) mesoporous xerogel powders are synthesized by co-crystallization of the corresponding nitrates with ultrasonic processing and used to obtain nanoscale ceramic materials with cubic fluorite-like, orthorhombic, and perovskite-like tetragonal crystal structure, respectively, with CSR ∼ 64–81 nm (1300°C). Physicochemical characterization of the obtained ceramics revealed that (СeO2)1-x(Sm2O3)x features with open porosity 2–6%, while for La1-xSrxNiO3, La1-xSrxCoO3, and La1-xSrxFe0.7Ni0.3O3, this value is about 21–29%. Ceria-based materials possess a predominantly ionic conductivity (ion transport numbers ti = 0.82–0.71 in the temperature range 300–700°C, σ700°С = 1.3·10−2 S/cm) determined by the formation of mobile oxygen vacancies upon heterovalent substitution of Sm3+ for Се4+. For solid solutions based on lanthanum nickelate and cobaltite, a mixed electronic-ionic conductivity (σ700°С = 0.80·10−1 S/cm) with ion transport numbers (te = 0.98–0.90, ti = 0.02–0.10) was obtained. The obtained ceramic materials are shown to be promising as solid oxide electrolytes and electrodes for medium-temperature fuel cells.

Thin films of SrCe0.95Y0.05O3 have been successfully prepared on Si(100) substrates using flame-assisted chemical vapor deposition. XRD characterization shows the inclusion of fluorite phase of CeO2 in the perovskite phases, which may be caused by strontium depletion during deposition. The effects of total-metal concentration in the solution and substrate temperature on the surface morphology and think thickness growth of the coatings have been explored. It was found that both the intermediate concentration and temperature produces smoothest films and increasing the concentration had more significant effect on enhancement of the film thickness growth.

Murataite-based ceramics containing 10 wt.% either UO2 or ThO2 were produced by inductive cold crucible melting (ICCM) at operating frequencies of 1.76 MHz and 5.28 MHz and examined using X-ray diffraction and scanning and transmission electron microscopy. High specific productivity was achieved. Average melting ratio was 8.3 kW×h/kg. We identified three distinct murataite polytypes in the U-bearing ceramics: a five- (5C), eight- (8C), and three-fold (3C) fluorite unit cell composing respectively the core, intermediate and rim zones of the grains. In contrast, in the Th-bearing ceramic one of the polytypes with eight-fold (8C) fluorite unit cell was found to be prevailing over two others (5C and 3C). Computer simulation of the major reflection due to the murataite phase in the Th-bearing sample also exhibits superposition of peaks due to three distinct polytypes but one of them (8C) is predominant. The core zone of the murataite in the U-bearing specimens is characterized by UO2 concentrations as high as 12.1 wt%, which successively diminishes in concentration through the intermediate zone to the rim, the latter of which contains 5.2 wt% UO2. Thorium distribution within the murataite crystals is more uniform. The other phases found in the ceramics are crichtonite, rutile and traces of perovskite, Fe/Mn titanate ilmenite/pyrophanite, zirconolite, and vitreous phase. The difference in phase composition and actinide partitioning in the ceramics is influenced by synthesis conditions. Thus, application of large-scale cold crucibles is a prospective route for the development of industrial-scale process and technology for ceramization of actinide-bearing HLW. The advantage of this method is production of zoned crystals with the highest concentrations of actinides and rare earth elements in the core, effectively isolating these elements from potential leach solutions.