From 2D to 3D Characterization of Materials Subjected to Extreme Pressure and Temperature Conditions

Planetesimal were the first planetary objects to form in the solar system, which later grew to make the proto-planets. Most of these bodies were differentiated as a result of internal heating. Several differentiated bodies have, then, been accreted following the giant impacts to create the terrestrial planets. As a result of these impacts, the newly formed Earth was molten and completely differentiated. Subsequent crystallization has given rise to Earth's current structure. In order to bring new constraints on the differentiation and melting relationship in the planets we have studied natural and synthetic samples corresponding to different stages of planetary evolution through state of the art electron microscopy techniques. We have, first, looked at carbonaceous materials in a ureilite meteorite (Almahata Sitta MS-170). Thin sections from ureilite diamonds were prepared with the focused ion beam (FIB) are observed by transmission electron microscopy and spectroscopy. The morphology of graphite bands in diamonds indicated that they result from the diamond to graphite transformation during a shock event. Moreover, the diamonds in this meteorite with crystallite sizes as large as ~20 ÎŒm can only form under static high-pressure condition of planetary interiors. We have, also, found three types of diamond inclusions in our samples. The majority of these inclusions are euhedral Fe-S inclusions. However, each of these inclusions has three phases, namely: kamacite (Fe, Ni), troilite (FeS), and schreibersite ((Fe, Ni)3 P). The chemical analysis of the intact inclusions shows them to be a stoichiometric phase, (Fe, Ni)3(S, P), that can only form above 21 GPa. The ureilite parent body (UPB) should have had the size about that of Mars to exert necessary pressure to grow the diamond inclusions in the core-mantle boundary. The other two types of inclusions are the Al- and Mg- free chromite, Cr2FeO4, and Ca-Fe phosphates that were previously observed only in iron meteorites. Our results suggest that the diamonds and their inclusions were formed from an S-rich metallic liquid. In the second part of the thesis, we have studied melting and fractional crystallization of the lower mantle using a laser-heated diamond anvil cell (LH-DAC). San Carlos olivine is used as a proxy to the mantle composition. The recovered samples are first analyzed with the 3D chemical tomography using a dual beam FIB instrument. The molten region in all the samples at pressure range from 30 to 71 GPa have at least three distinct zones: a ferropericlase shell (Fp), an intermediate bridgmanite (Brg) region and the Fe-rich melt core. Thin sections from the center of the same samples are analyzed with TEM and energy-dispersive x-ray (EDX) spectroscopy. The results from the samples heated at 45 GPa for 1, 3, and 6 minutes demonstrated that the temperature gradient in the heated zone shrinks through the time and, thus, the crystallization continues toward the center of the heating. Consequently, the melt becomes richer in iron. The melt core also gets more iron-rich with increasing pressure. In fact, in ~70 GPa we observe an Fe-O core with small Si and Mg concentration. This implies that the melt at the bottom of the mantle could become denser than the solid phases and sink down. The presence of the iron-rich melt or the oxides crystallizing from such a melt can explain the ultra-low velocity zones (ULVZs) found in the seismic surveys of the core-mantle boundary.

Chemical Characterisation of Deep Earth Reservoirs

Hélène Marie Piet

Seismic imaging of the internal Earth has emphasized the inhomogeneity of its mantle. This interface layer of nearly 3000 km surrounding the central metallic core conducts heat released from the latter, which played a significant role in shaping the actual mantle as it currently is. The present state of the deep Earth (mainly the lower mantle and the core) is also a consequence of the physical properties of the material composing them. Large-low shear velocity provinces atop the core-mantle boundary, for example, are composed of a denser material than their surrounding environment but their origin, formation and survival over billions of years to mantle convection, is still largely debated. Here we use chemical partitioning to characterise the current composition of those deep Earth reservoirs and better understand the processes leading to their formation. In the first part, experiments were conducted in the laser-heated diamond anvil cell and the latest advances in microanalysis were used to refine iron partitioning between minerals of the lower mantle. We reconcile long-standing discrepancies in previous partitioning data and show that significant variations of silicate iron content are associated to changing ferric iron incorporation. We also note that such chemical variations might significantly affect the physical properties of the silicate, which might in turn provoke mantle rheology variations. Our results and observations thus provide a mineral physics based argument for the stabilization of deep Earth reservoirs and especially large structures like large low-shear-velocity provinces (LLSVPs). In the second and numerical part of the thesis, previous results on siderophile (iron-loving) element partitioning between metal and silicate were used to constrain some aspects of core formation. We constrain the chemical and physical state of the magma ocean during core formation explaining both the siderophile composition of the mantle and the light element composition of the core. We simulate the Moon-Forming Giant Impact in the last steps of Earth accretion, where we constrain the size of the impactor to be smaller than 15% of Earth's total mass, a roughly Mars-sized object, and further show that melting of the large fractions of the mantle upon impact by the giant impactor is consistent with siderophile composition of the bulk silicate Earth, which can have great implications for the geodynamical evolution of the mantle. In conclusions, given the fact that the mantle may have been almost fully molten, further crystallization may have isolated a layer of liquid primitive material at the base of the mantle possibly explaining the seismic signature of the LLSVPs associated with a strong thermochemical contrast with the surrounding mantle. The layered composition in iron of the actual mantle also provides a chemical argument for a viscously layered mantle, which could have helped sustain LLSVPs for billions of years from efficient convection.

EPFL2016

A large planetary body inferred from diamond inclusions in a ureilite meteorite

James Badro, Marco Cantoni, Teresa Katharina Dennenwaldt, Philippe Gillet, Cécile Hébert, Farhang Nabiei, Emadeddin Oveisi

Planetary formation models show that terrestrial planets are formed by the accretion of tens of Moon-to Mars-sized planetary embryos through energetic giant impacts. However, relics of these large proto-planets are yet to be found. Ureilites are one of the main families of achondritic meteorites and their parent body is believed to have been catastrophically disrupted by an impact during the first 10 million years of the solar system. Here we studied a section of the Almahata Sitta ureilite using transmission electron microscopy, where large diamonds were formed at high pressure inside the parent body. We discovered chromite, phosphate, and (Fe,Ni)-sulfide inclusions embedded in diamond. The composition and morphology of the inclusions can only be explained if the formation pressure was higher than 20 GPa. Such pressures suggest that the ureilite parent body was a Mercury-to Mars-sized planetary embryo.

2018

Advanced analytical electron microscopy applied to Solid Oxide Cell materials and their degradation

Stéphane François Jean Poitel

Electron microscopy offers a wide range of techniques to study materials. Environmental Scanning Electron Microscopy (ESEM), in particular, allows the characterisation of samples at high-temperature using a laser heating stage, and under various gaseous atmospheres leading to direct observation of the sample surface morphology dynamics in conditions that are close to those in real operating devices (i.e. at high temperature in various gases). Solid Oxide Fuel Cells (SOFC) operating around 800°C in both oxidising and reducing gases, with proven co-generation efficiencies of 90%, offer very promising solutions in the ongoing energy transition. However, production costs and materials degradation in those demanding operating conditions remain significant barriers to their large-scale deployment. Studying SOFC materials' degradation through a combination of ESEM with other advanced microscopy techniques fulfilled the double aim of demonstrating the power of an ESEM and improving the understanding of SOFC materials' degradation during operation. Two materials from the SOFC stack were selected for degradation case studies at around 800°C in oxidising/reducing gases: coated ferritic steel interconnect and Ni-YSZ anode. The objectives were to explore the parameters' influence (temperature, heating ramp, pressure, gas exposure...), set the boundaries and define the guidelines of the ESEM technique while demonstrating its capabilities. The two different materials sets were selected for their relevance both for the SOFC field and for their different specific characteristics allowing a wide range of experiments to be explored. A specific method for the study of the oxidation of coated steel was developed combining ESEM, Focused Ion Beam (FIB) and Scanning Transmission Electron Microscopy (STEM). Post-test segmentation of the in-situ observed images demonstrated the possibility to obtain quantitative data. FIB sample preparation from the exact site of ESEM observation allowed to correlate the underlying microstructure with the in-situ surface observation. Applying this method, oxidation mechanisms of several coatings were characterised and specific behaviours observed and explained such as the appearance of oxide ridges and the behaviour of a cerium barrier layer in case of cobalt-cerium coated steel. The heating ramp speed and the oxidation temperature were demonstrated to be crucial parameters to obtain the desired microstructure. The nitriding pretreatment of interconnect was investigated too and shows promising results for improving the steel oxidation resistance. Another combination of techniques was developed for the study of the Ni-YSZ anode by complementing the ESEM observation with continuous Mass Spectrometer (MS) acquisition. The first reduction of the NiO-YsZ anode was analysed under gas flows of hydrogen and hydrocarbons. Redox cycling effect on the anode microstructure was investigated and the resulting observations were in line with models from the literature for the reduction and re-oxidation kinetics. Finally, the demonstration of an active solid oxide cell in the microscope operating at more than 700°C under mixed gas paves the way to further possible studies approaching the device's observation in real operating conditions. Limitation of the ESEM techniques are discussed and recommendations for further improvements and perspectives given.