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.
| Identifer | oai:union.ndltd.org:ADTP/216727 |
| Date | January 2001 |
| Creators | Schmidt, Marek, Wojciech, Marek.Schmidt@rl.ac.uk |
| Publisher | The Australian National University. Research School of Physical Sciences and Engineering |
| Source Sets | Australiasian Digital Theses Program |
| Language | English |
| Detected Language | English |
| Rights | http://www.anu.edu.au/legal/copyright/copyrit.html), Copyright Marek, Wojciech Schmidt |
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