Spelling suggestions: "subject:"ballmilling"" "subject:"sawmilling""
91 |
Électrolytes solides fluorés pour batteries tout solide à ions F- / Fluoride solid electrolytes for Fluoride Ion BatteryChable, Johann 30 November 2015 (has links)
Ce travail porte sur la synthèse, la mise en forme et la caractérisation desolutions solides de type tysonite RE1-xMxF3-x (RE = La, Sm, Ce et M = Ba, Ca, Sr). Dans unpremier temps, une démarche d’étude rigoureuse est mise en place pour la solution solide ditede référence, La1-xBaxF3-x. Les synthèses menées à l’état solide aboutissent à une maîtrise dela composition chimique et à l'établissement de lois de variations des paramètres structuraux.Une meilleure compréhension de l’influence de la structure sur la mobilité des ions F- estégalement acquise. L’influence du frittage dans l’obtention de bonnes valeurs de conductivitéionique est également à souligner. Dans un second temps, les effets de la nanostructurationpar mécanobroyage sur les propriétés de conductivité sont évalués. L’utilisation de laméthodologie des plans d’expériences mène à la mise au point des réglages optimums debroyage. Il apparaît alors que la synthèse des électrolytes peut être accélérée et mise àl’échelle tout en gardant des valeurs optimales de conductivité. Enfin, la démarche déterminéeest appliquée à d'autres solutions solides de type tysonite et à la recherche du conducteurionique le plus performant. Si les composés issus de la substitution Ce/Sr ou encore Sm/Casemblent les plus prometteurs, la plus grande stabilité chimique de la solution solide La1-xSrxF3-x est le compromis idéal pour l'utiliser comme électrolyte solide lors des mesuresélectrochimiques des batteries. / This work deals with the synthesis, shaping and characterization of RE1-xMxF3-x (RE = La, Sm, Ce et M = Ba, Ca, Sr) tysonite-type solid solutions. In a first part, onemeticulous approach has been set up for La1-xBaxF3-x solid solution, chosen as a reference.The solid-state synthesis of these materials led to a better knowledge of their chemicalcomposition (Vegard’s laws) and of the structure-ionic mobility correlations. The impact ofthe sintering process on the ionic conductivity is also highlighted. In a second part, the effectsof the nanostructuration conducted by ball-milling of the microcrystalline samples areevaluated. The use of the Design of Experiments methodology led to identify the optimummilling conditions. It appears that the synthesis of electrolytes can be sped- and scaled-up,while keeping high ionic conductivity properties. At last, this approach is applied on othertysonite-type solid solutions, to look for the best electrolyte. The Ce/Sr and Sm/Casubstitutions generate very promising ionic conductors but not really (electro)chemicallystable compounds. A compromise has been found with the choice of the La1-xSrxF3-x solidsolution as the FIB electrolyte for the electrochemical performances tests, regarding its higherchemical stability.
|
92 |
Phase formation and structural transformation of strontium ferrite SrFeOxSchmidt, Marek, Wojciech, Marek.Schmidt@rl.ac.uk January 2001 (has links)
Non-stoichiometric strontium iron oxide is described by an abbreviated formula SrFeOx (2.5 ≤ x ≤ 3.0) exhibits a variety of interesting physical and chemical properties over a broad range of temperatures and in different
gaseous environments. The oxide contains a mixture of iron in the trivalent and the rare tetravalent state. The material at elevated temperature is a mixed oxygen conductor and it, or its derivatives,can have practical
applications in oxygen conducting devices such as pressure driven oxygen
generators, partial oxidation reactors in electrodes for solid oxide fuel cells
(SOFC).
¶
This thesis examines the behaviour of the material at ambient and elevated temperatures using a broad spectrum of solid state experimental
techniques such as: x-ray and neutron powder diffraction,thermogravimetric and calorimetric methods,scanning electron microscopy and Mossbauer
spectroscopy. Changes in the oxide were induced using conventional thermal
treatment in various atmospheres as well as mechanical energy (ball milling).
The first experimental chapter examines the formation of the ferrite from
a mixture of reactants.It describes the chemical reactions and phase transitions that lead to the formation of the oxide. Ball milling of the reactants prior to annealing was found to eliminate transient phases from the reaction route and to increase the kinetics of
the reaction at lower temperatures.
Examination of the thermodynamics of iron oxide (hematite) used for the
reactions led to a new route of synthesis of the ferrite frommagnetite and
strontium carbonate.This chapter also explores the possibility of synthesis
of the material at room temperature using ball milling.
¶
The ferrite strongly interacts with the gas phase so its behaviour was studied under different pressures of oxygen and in carbon dioxide.The changes in ferrite composition have an equilibrium character and depend on temperature and oxygen concentration in the
atmosphere. Variations of the oxygen
content x were described as a function of temperature and oxygen partial
pressure, the results were used to plot an equilibrium composition diagram.
The heat of oxidation was also measured as a function of temperature and oxygen partial pressure.
¶
Interaction of the ferrite with carbon dioxide below a critical temperature
causes decomposition of the material to strontium carbonate and SrFe12O19 .
The critical temperature depends on the partial pressure of CO2 and above
the critical temperature the carbonate and SrFe12O19 are converted back into
the ferrite.The resulting SrFe12O19 is very resistant towards carbonation and
the thermal carbonation reaction does not lead to a complete decomposition
of SrFeOx to hematite and strontium carbonate.
¶
The thermally induced oxidation and carbonation reactions cease at room
temperature due to sluggish kinetics however,they can be carried out at ambient temperature using ball milling.The reaction routes for these processes are different from the thermal routes.The mechanical oxidation induces two
or more concurrent reactions which lead to samples containing two or more
phases. The mechanical carbonation on the other hand produces an unknown
metastable iron carbonate and leads a complete decomposition of the ferrite
to strontiumcarbonate and hematite.
¶
Thermally and mechanically oxidized samples were studied using Mossbauer
spectroscopy. The author proposes a new interpretation of the Sr4Fe4O11
(x=2.75) and Sr8Fe8O23 (x=2.875)spectra.The interpretation is based
on the chemistry of the compounds and provides a simpler explanation of
the observed absorption lines.The Mossbauer results froma range of compositions
revealed the roomtemperature phase behaviour of the ferrite also
examined using x-ray diffraction.
¶
The high-temperature crystal structure of the ferrite was examined using
neutron powder diffraction.The measurements were done at temperatures
up to 1273K in argon and air atmospheres.The former atmosphere protects
Sr2Fe2O5 (x=2.5) against oxidation and the measurements in air allowed
variation of the composition of the oxide in the range 2.56 ≤ x ≤ 2.81.
Sr2Fe2O5 is an antiferromagnet and undergoes phase transitions to the paramagnetic
state at 692K and from the orthorhombic to the cubic structure
around 1140K.The oxidized formof the ferrite also undergoes a transition
to the high-temperature cubic form.The author proposes a new structural
model for the cubic phase based on a unit cell with the Fm3c symmetry.
The new model allows a description of the high-temperature cubic form of
the ferrite as a solid solution of the composition end members.The results
were used to draw a phase diagramfor the SrFeOx system.
¶
The last chapter summarizes the findings and suggests directions for further research.
|
Page generated in 0.0682 seconds