Rozprawy doktorskie na temat „Oxygen Ion Conductors”

Dense ceramic membranes with mixed ionic and electronic conductivity can separate 100 % pure oxygen from air at elevated temperature. They constitute a much cheaper and more environmentally friendly alternative to cryogenic distillation. Today the challenge with dense ceramic membranes is the long term stability. The high operating temperatures (>800°C) of the membranes lead to chemical degradation and mechanical failure. Hexagonal manganites has the ability to reduce the problems associated with high temperatures. Compared to perovskites, which conduct through formation of oxygen vacancies, hexagonal manganites have the ability to form interstitial oxygen due to a less closed packed structure. In this thesis, it is reported on how the structure and oxygen storage ability changes with varying yttrium deficiency in hexagonal YMnO3. It is important to investigate the possibility of cation vacancies in the structure, as little is known about the defect chemistry of these materials. Yttrium deficient samples of YMnO3 made by solid state synthesis was fired in air, and annealed in varying atmospheres. Hexagonal YMnO3 has the ability to lose 15-20 % yttrium without any notable changes in the structure and composition of yttrium deficient YMnO3. Above this limit, reflections from Mn3O4 hausmannite appears in x-ray diffractograms. With a higher degree of yttrium deficiency in the hexagonal structure prepared in air, a slight contraction of the unit cell along both a and c direction is found. After annealing in O2 atmosphere, the structure expands along the a axis and contracts along the c-axis. The opposite behavior is seen in inert (N2) atmosphere, where the structure expands in c direction and contract along the a axis with increased yttrium deficiency in YMnO3. Above 600 ˚C, during heating of YMnO3 and Y0.80MnO3, oxygen goes out of the materials in both N2 and O2 atmospheres. Upon cooling in O2 atmosphere, a reversible oxygen uptake is seen around 400 ˚C for both YMnO3 and yttrium deficient YMnO3. For stoichiometric YMnO3 the reversible oxygen adsorption is suggested to be attributed to interstitial oxygen, while for stoichiometric yttrium deficient YMnO3 it is believed to be filling of oxygen vacancies. The reversible oxygen uptake of YMnO3 was calculated to be δ= 0.034. Moreover, in this thesis it is also reported on the synthesis of an asymmetric membrane consisting of 15 % titanium substituted YMnO3. The asymmetric membrane did not yield a dense functional layer of nanocrystalline YMn0.85Ti0.15O3 on top of the porous support after sintering.

In this thesis, the focus is on studying BaZrO3-BaCeO3 based proton conductors due to that they represent very promising proton conductors to be used for Intermediate Temperature Solid Oxide Fuel Cells (ITSOFCs). Here, dense BaZr0.5Ce0.3Y0.2O3-δ (BZCY532) ceramics were selected as the major studied materials. These ceramics were prepared by different sintering methods and doping strategies. Based on achieved results, the thesis work can simply be divided into the following parts: 1) An improved synthesis method, which included a water-based milling procedure followed by a freeze-drying post-processing, was presented. A lowered calcination and sintering temperature for a Hf0.7Y0.3O2-δ (YSH) compound was achieved. The value of the relative density in this work was higher than previously reported data. It is also concluded that this improved method can be used for mass-production of ceramics. 2) As the solid-state reactive sintering (SSRS) represent a cost-effective sintering method, the sintering behaviors of proton conductors BaZrxCe0.8-xLn0.2O3-δ (x = 0.8, 0.5, 0.1; Ln = Y, Sm, Gd, Dy) during the SSRS process were investigated. According to the obtained results, it was found that the sintering temperature will decrease, when the Ce content increases from 0 (BZCLn802) to 0.3 (BZCLn532) and 0.7 (BZCLn172). Moreover, the radii of the dopant ions similar to the radii of Zr4+ or Ce4+ ions show a better sinterability. This means that it is possible to obtain dense ceramics at a lower temperature. Moreover, the conductivities of dense BZCLn532 ceramics were determined. The conductivity data indicate that dense BZCY532 ceramics are good candidates as either oxygen ion conductors or proton conductors used for ITSOFCs. 3) The effect of NiO on the sintering behaviors, morphologies and conductivities of BZCY532 based electrolytes were systematically investigated. According to the achieved results, it can be concluded that the dense BZCY532B ceramics (NiO was added during ball-milling before a powder mixture calcination) show an enhanced oxygen and proton conductivity. Also, that BZCY532A (NiO was added after a powder mixture calcination) and BZCY532N (No NiO was added in the whole preparation procedures) showed lower values. In addition, dense BZCY532B and BZCY532N ceramics showed only small electronic conductivities, when the testing temperature was lower than 800 ℃. However, the BZCY532A ceramics revealed an obvious electronic conduction, when they were tested in the range of 600 ℃ to 800 ℃. Therefore, it is preferable to add the NiO powder during the BZCY532 powder preparation, which can lower the sintering temperature and also increase the conductivity. 4) Dense BZCY532 ceramics were successfully prepared by using the Spark Plasma Sintering (SPS) method at a temperature of 1350 ℃ with a holding time of 5 min. It was found that a lower sintering temperature (< 1400 ℃) and a very fast cooling rate (> 200 ℃/min) are two key parameters to prepare dense BZCY532 ceramics. These results confirm that the SPS technique represents a feasible and cost-effective sintering method to prepare dense Ce-containing BaZrO3-BaCeO3 based proton conductors. 5) Finally, a preliminary study for preparation of Ce0.8Sm0.2O2-δ (SDC) and BZCY532 basedcomposite electrolytes was carried out. The novel SDC-BZCY532 based composite electrolytes were prepared by using the powder mixing and co-sintering method. The sintering behaviors, morphologies and ionic conductivities of the composite electrolytes were investigated. The obtained results show that the composite electrolyte with a composition of 60SDC-40BZCY532 has the highest conductivity. In contrast, the composite electrolyte with a composition of 40SDC-60BZCY532 shows the lowest conductivity. In summary, the results show that BaZrO3-BaCeO3 based proton-conducting ceramic materials represent very promising materials for future ITSOFCs electrolyte applications.

QC 20150423

This study investigates the efficiency of a calcia stabilised zirconia (CaSZ) solid electrolyte as an oxygen ion conductor. The study also examines the behaviour of the oxygen species conducted by the solid electrolyte compared to species provided in the gas phase for partial oxidation of hydrocarbons. In this work, an electrochemical cell of the form Air, AgHCaSZ//Ag, Carrier gas was used to investigate the electrochemical efficiency and stability of the solid electrolyte CaSZ conducting of oxygen ions under atmospheric pressure conditions at 500 degrees C by applying a range of electrical potentials from I to 16 volts across the electrochemical cell. Due to the applied potential oxygen anions are transferred across the solid electrolyte from the cathode side of the cell to the anode side. It was found that the employed electrolyte is approximately a 100% purely ionic conductor of oxygen ions in the range of electrical voltage applied from I to 10 volts. Above that range the cell started to degrade and loose its ionic efficiency. It was possible to generate gas mixtures containing trace quantities of oxygen. The viscosity of these gas mixtures as a function of oxygen concentration was determined using an established flow perturbation technique (Flux Response Technology). Partial oxidation of propene was used to investigate the difference between the oxygen species produced electrochemically via electrical potential application across the electrochemical cell Air, AgHCaSZ//Ag, Propene, Ar and oxygen provided in the gaseous state co-fed with propene over silver electrode under atmospheric pressure and 450 degrees C and 500 degrees C. It was found that the method of electrochemical provision of oxygen caused the silver catalyst to be more selective to 1,5-hexadeine, whereas the gaseous oxygen provision produced acrolein as the major product. Carbon dioxide formation was not affected by the method of oxygen provision. The Ag electrode was compared to an Au-rich Ag alloy electrode for propene partial oxidation using electrochemical provision. It was found that 1,5-hexadiene was the major product over both electrodes, but the Au-rich alloy was more selective for acrolein than the Ag electrode. This might be due to the gold serving as a separator between Ag particles which hinder the back-spill over of oxygen and allow desorption of molecular oxygen in the gas phase, which then re-adsorb molecularly on silver sites producing acrolein. The effect of the sequence of the method of oxygen provision on the partial oxidation of propene was tested using the electrochemical cell Y-BiMoHAg//CaSZ//Ag at 450 degrees C and atmospheric pressure. A sharp decrease in acrolein selectivity was found when oxygen was provided in the gas phase after treatment with electrochemical oxygen, while no significant effect was noticed when the electrochemical oxygen was used after treatment with gaseous oxygen. This large decrease in acrolein selectivity might be attributed to the severe reduction of the catalyst, which is probably caused by high electrical potential application. A temperature increase from 450 to 500 degrees C seemed to suppress the formation of acrolein for both methods of oxygen provision and enhance the 1,5-hexadiene formation.

L'etude porte sur la variation de la conductivite electrique des systemes borotellurates et borophosphates suivants : lio::(2)-b::(2)o::(3)-te::(2)o::(4) et li::(2)o-b::(2)o::(3)-p::(2)o::(6)-lix (x=f, cl, br). L'effet de formateur mixte pour le premier systeme et l'effet de sel dopant dans le second systeme ont ete interpretes a l'aide de la theorie de l'electrolyte faible et d'un modele de solutions regulieres

Les conducteurs d'ions oxygène fonctionnant à des températures inférieures à 300 ° C sont des matériaux d'intérêt majeur pour une série d’applications technologiques telles que les piles à combustible solide, les batteries, les électrodes, les capteurs, des catalyseurs, etc. Cependant à l’heure actuelle, les conducteurs d'ions d'oxygène solides fonctionnent raisonnablement seulement à haute température, supérieure à 800°C, ce qui limite leur application. Dans la recherche de l'amélioration des conducteurs d'ions d'oxygène, la structure Brownmillérite (ABO2.5 éq. A2B2O5) a toujours joué un rôle important, en particulier dans le régime à basse température où la dynamique de la chaîne tétraédrique induit la mobilité de l'oxygène. Dans ce contexte, nous avons synthétisé une nouvelle phase Sr2-xBaxScGaO5 (avec x=0 SSGO et x= 0.1 SBSGO), contenant des ions 3d0 diamagnétiques et susceptible d’être un conducteur ionique pur. En fonction de la voie de synthèse, le composé présente deux polymorphes, orthorhombiques et cubiques, qui sont tous deux importants pour la conductivité de l'oxygène. La réaction à l’état solide conduit à une structure de type Brownmillerite orthorhombique tandis que la synthèse de fusion de zone (FTZ) donne une structure Pérovskite déficitaire en oxygène .Par diffraction neutronique sur poudre (D2B @ ILL) nous avons analysé la structure des deux polymorphes, en fonction de la température. Une analyse détaillée du type SSGO Brownmillerite montre que le Sc occupe les sites octaédriques, tandis que Ga occupe exclusivement les tétraèdres autres. Cet ordre de cations est assez inhabituel pour les structures de type Brownmillerite. La deuxième particularité est que Sr2-xBaxScGaO5 subit une transition de phase à partir d'une configuration ordonnée des chaines (GaO4), caractéristiques du groupe d’espace I2mb à température ambiante, vers une configuration désordonnée des chaînes dans le groupe d’espace Imma (à 500°C). Ce résultat important confirme notre hypothèse que le désordre est dynamique et il est la clé pour avoir un conducteur d'ions d'oxygène à températures modérées. La synthèse à des températures élevées (jusqu'à fusion), donne une structure cubique Pm ̅m, stable jusqu'à 1000 ° C. La structure est de type Pérovskite fortement déficitaire en oxygène. La mobilité de l’oxygène de ces nouveaux composés a été ensuite étudiée par la thermogravimétrie (TGA) couplée avec spectroscopie de masse (MS) après échange isotopique 18O-16O, par spectroscopie RAMAN et RMN couplée avec les calculs théoriques ab-initio (WIEN2k), par diffusion inélastique des neutrons (IN6@ILL) couplée avec des calculs de dynamiques moléculaire ab-initio (VASP). Les résultats obtenus via les études structurales et de dynamique de réseau montrent que l’activation de la mobilité ionique est liée à la transition vers la structure désordonnée Imma, qui implique une dynamique importante des chaines GaO4 et une diffusion unidimensionnel le long des canaux lacunaires. Ces résultats ont pu être reproduits par calculs de dynamique moléculaire, dans lesquels la diffusion ne concerne que les oxygènes des plans tétraédriques, et s’expliquent par des paramètres de maille a et c qui sont significativement augmentés par rapport à (Ca/Sr)FeO2.5
Oxygen ion conductors operating at low temperature, below 300 ° C, are materials of major interest for several applications in the area of solid state ionicsas solid fuel cells, batteries, electrodes, sensors, catalysts, etc. However till now, the solid oxygen ion conductor works reasonably only at high temperatures above 800 ° C, which limits their application. In the search for improved oxygen ion conductors Brownmillerite structures ( ABO2.5 eq. A2B2O5 ) has always played an important role, especially in the low temperature regime where the dynamics of the tetrahedral chain induced mobility of oxygen. In this context, we have synthesized a new phase Sr1-xBaxScGaO5 with x = 0 (SSGO) and x = 0.1 (SBSGO) containing diamagnetic 3d0 ions to have a pure ion conductor. Depending on the synthesis route, the compound has two polymorphs, orthorhombic and cubic, which are both important for the oxygen conductivity. The reaction in the solid state leads to an orthorhombic Brownmillerite-type structure, while tmeling synthesis (using the Travelling Floating Zone method FTZ ) gives an oxygen-deficient Perovskite structure. The structures of both polymorphs were analyzed using the neutron powder diffraction as function of the temperature (D2B@ILL). A detailed analysis of SSGO Brownmillerite type shows that the Sc occupies octahedral sites, while the Ga occupies exclusively the tetrahedral ones. This cation ordering is unusual for the Brownmillerite structures. Moreover Sr2-xBaxScGaO5 undergoes a phase transition from an ordered configuration of the tetrahedral chains (GaO4) characteristic of I2mb space-group at room temperature, toward a disordered one characteristic of Imma space group (500 ° C). This important result confirms that the disorder of the tetrahedral chains is dynamic and it is the key to have oxygen ion conductor at moderate temperatures. Synthesis at elevated temperatures (up to melting point) gives a cubic structure Pm ̅m, stable up to 1000 ° C. The Perovskite -type structure is highly oxygen deficient. The mobility of the oxygen of these new compounds was studied by thermogravimetry analysis (TGA) coupled with mass spectrometry (MS) after the isotope exchange 18O-16O, by Raman and NMR spectroscopy coupled with theoretical ab-initio calculations (WIEN2k), by inelastic neutron scattering (IN6@ILL) coupled with calculations of ab-initio molecular dynamics (VASP ) . The results obtained from the structural and the lattice dynamics studies show that activation of the ion mobility is related to the transition to a disordered structure Imma, which implies an important dynamics of the chains GaO4 and the diffusion along the one-dimensional vacancy channel. These results have been reproduced by molecular dynamics calculations, in which the diffusion pathway is due only to the oxygen in the tetrahedral planes

De nouveaux conducteurs anioniques dérivés de Nd4Ga2O9, Nd4GeO8 et Nd3GaO6, ont été obtenus par substitutions cationiques. Les oxy-Cuspidine Nd4[Ga2(1-x)M2xO7+xٱ1-x]O2 (M=Ge,Ti) ont été préparées par diverses techniques. Avec la substitution, la structure Cuspidine est maintenue jusqu'à x=0,5 et 1,0 pour M=Ge et Ti respectivement. Dans les deux cas, une modulation 1D de la structure est observée pour x  0,15 mais l'effet du substituant sur la conductivité anionique est différent : Ti joue un rôle assez neutre alors que Ge l'améliore. De nouveaux composés ont aussi été préparés par substitution de Ge4+ par Ga3+ dans Nd4GeO8, de Ga3+ par Zn2+, Mg2+, et de Nd3+ par Ca2+, Sr2+ dans Nd3GaO6. Une technique d'auto-combustion a été développée pour synthétiser ces matériaux à 900°C. Il est possible de créer 10% de lacunes d'oxygène dans Nd4GeO8 et moins de 1% dans Nd3GaO6. Cependant, ces taux de substitution relativement faibles induisent une nette augmentation de la conductivité.

Le chapitre 1 présente le contexte des piles à combustibles et des conducteurs par ions 02-@ puis les techniques expérimentales utilisées, dont la spectroscopie d'impédance complexe. Le chapitre Il est dédié à l'étude de l'effet des substitutions de La par R=Nd,Gd,Y et de Mo par W sur les propriétés de La2MO209, nouveau conducteur par ions 02-. L'effet stabilisateur de W et destabilisateur de R sur la réductibilité du molybdate est montré. La sensibilité de la conduction aux conditions de frittage est abordée au chapitre 111, s'ouvrant sur l'étude de la densification des phases par broyage et frittage optirnisé par dilatométrie. L'influence de la porosité sur la conduction est interprétée en terme d'effet de surface, et un comportement de type VTF est postulé à haute T. La forte mobilité anionique de ces phases est montrée par mesure de coefficient de diffusion de traceurs. Le chapitre IV explore le potentiel du concept LPS pour la découverte de nouveaux conducteurs avioniques
Chapter 1 presents the thesis context relative to fuel cells and fast oxide-ion conductors, together with the various techniques used, among which complex irnpedance spectroscopy. Chapter Il is devoted to the study of the substitution effect of La by R=Nd,Gd,Y and of Mo by W on the properties of La2Mo2O9, a new fast oxide-ion conductor. The stabilizing effect of W and destabilizing effect of R on the molybdate reducibility is dernonstrated. The sensitivily of conduction to sintering conditions is explored in chapter In, which opens up on a study of the phases densification through milling, then sintering as optirnised through dilatometry. The influence of porosity on conduction is interpreted in terni of surface effect, and a VTF-type behaviour is postulated at high T. The high anion mobility of these phases is evidenced through tracer diffusion coefficient rneasurements. Chapter IV explores the possibility to use the so-called LPS concept to discover new oxide-ion conductors

De nouveaux verres, conducteurs ioniques de l' ion Ag+, ont été mis en évidence dans les systèmes TeO2-Ag2O-AgI et TeO2-Tl2O-AgI. Les caractérisations structurales (IR, Raman et RMN 109Ag) ont révèle l'existence de deux sous-réseaux, l'un iode, l'autre oxygène. L'étude des propriétés électriques en fonction de la composition montre que la conduction ionique est principalement liée a la mobilité des ions Ag+ situes dans un environnement iode. La substitution de AgI par la composition eutectique (AgI0,75TlI0,25) permet d'élargir le domaine vitreux et d'obtenir des verres présentant de très bonnes performances électriques. La présence de TlI semble induire l' existence de chemins de conduction plus ouverts facilitant ainsi un meilleur déplacement des ions Ag+.

Les propriétés de conduction par ions oxydes de solutions solides Bi1-xMxO1,5-x/2 (M = Ca, Sr, Ba) déficitaires en oxygène ont fait l'objet d'investigations. La conductivité ơ a été mesurée par la méthode des impédances complexes et le nombre de transport ionique de l'oxygène ti évalué au moyen d'une pile de concentration Air, Au/ Bi2O3-MO / Au, O2 (1 atm). L'allure de la conductivité en fonction de la température, caractérisée par une augmentation importante et réversible de ơ à une température Tt (Tt = f(x, M)) et du nombre de transport (ti # 1 pour T > Tt), a conduit à la mise en oeuvre de déterminations structurales par diffraction X sur monocristal : - à température ambiante : la structure de la variété la moins conductrice est constituée de feuillets séparés par des plans de conduction ; - en fonction de la température : l'analyse des résultats montre que le saut de conductivité s'accompagne d'une augmentation du nombre d'ions oxydes participant au phénomène de conduction. L'évolution de l'énergie d' activation en fonction de T ou de x est analysée. L'origine des deux maxima observés sur la courbe Ea = f(x), à T < Tt, lorsque M = Sr, est discutée à la lumière des résultats obtenus par diffraction électronique sur des échantillons de différentes compositions. Une interprétation est proposée qui fait appel à l'apparition de deux types d'ordre au sein du réseau cationique.

Mise en évidence d'une solution solide Bi(1-x)Cd(x)O(1,5-x/2) avec 0,111