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Chemomechanics of Non-stoichiometric Oxide Films for Energy Conversion
Başlık:
Chemomechanics of Non-stoichiometric Oxide Films for Energy Conversion
Yazar:
Swallow, Jessica G., author.
Yazar Ek Girişi:
Genel Not:
Source: Dissertation Abstracts International, Volume: 79-10(E), Section: B.
Advisors: Krystyn J. Van Vliet.
Özet:
Electrochemical energy conversion and storage devices including solid oxide fuel cells (SOFCs) and lithium ion batteries (LIBs) are enabled by materials known as "non-stoichiometric oxides" that contain large concentrations of point defects such as oxygen or lithium vacancies. While this non-stoichiometry provides the essential functional properties of ionic conductivity or reactivity that make these materials useful, it also tends to couple to material volume through the effect of chemical expansion. Chemical expansion, or volume coupled to defect concentration, is in turn tied to mechanical variables including stress, strain, and elastic constants. This electrochemomechanical coupling, or interaction between functional properties, defect chemistry, and mechanical variables, can have important consequences for devices operated in extreme environments, where unexpected stress may lead to fracture, or well-engineered strain may enhance device efficiency. Such effects are particularly important in thin film devices, where strain engineering is within reach, undesired fracture can devastate performance, and defect chemistry and related properties can differ from bulk systems. In this thesis, we present a concerted investigation of chemomechanical coupling, including interactions between material chemistry, environmental conditions. stress, strain, and mechanical properties, for films of the model material PrxCe1--x.O 2--8 (PCO) that is a fluorite-structured oxide relevant to SOFC applications. PCO is an excellent model system because of its well-established defect chemistry model and known thermal and chemical expansion coefficients. The thesis begins by first characterizing key chemomechanical effects in PCO, including electrochemically induced high temperature actuation and nonstoichiometry-dependent mechanical properties that are modulated by environmental conditions including temperature and oxygen partial pressure. We then explore the mechanisms and microstructural contributions to these effects via computational modeling and high temperature transmission electron microscopy, identifying ways in which chemomechanical effects in thin film non-stoichiometric oxides differ from those in bulk. Finally, we extend the experimental and computational methods developed in the thesis to characterizing similar effects in Li-storage materials, demonstrating the broad applicability of results across the classes of non-stoichiometric oxides.
We first describe an experimental study in which we developed a novel method of detecting chemical expansion on short time scales in the model system PCO and characterized material deformation for a range of conditions of temperature and effective oxygen partial pressure (pO2). In this method, electrically-stimulated chemical expansion caused mechanical deflection of a substrate, an effect that for PCO was enhanced for elevated temperatures or reducing conditions. We discuss the film and substrate properties that contributed to this high temperature oxide actuation, and consider methods of tuning this mechanical deflection. Additionally, we characterized the effect of high-temperature actuation for a perovskite-structured oxide (SrTi0.65Fe0.35 O3--8, STF) also demonstrating the applicability of the methods used beyond the model system PCO. This first study constitutes a demonstration of how chemical expansion can directly generate stress or strain that can produce substantial deformation in situ for layered devices based on non-stoichiometric oxide films.
Next, we explore the effect of thermal and chemical expansion on the mechanical stiffness of PCO films using high temperature, controlled atmosphere nanoindentation. We find that the Young's elastic modulus E of PCO decreases with increased temperature or decreased pO2 as a result of thermal and chemical expansion. Furthermore, the decrease in E observed for PCO films in situ is larger than would be expected based on bulk models or previous measurements of related oxides, demonstrating a new example of the ways in which chemomechanical coupling effects can differ between bulk and thin film forms of non-stoichiometric oxides operando.
In conjunction with the experimental study of the relationship between chemical expansion and E, we also applied density functional theory (DFT) to compute how changing defect chemistry and associated chemical expansion affect the elastic constants of PCO. Bulk computations reproduced the expected dependence of E on lattice parameter previously observed for bulk oxides in the literature. Additional simulations that directly compared bulk and membrane forms of PCO found that the biaxial stiffness of both forms should decrease with oxygen loss, but that this change was comparable for comparable changes in composition. These computational results provide additional evidence for understanding the mechanisms by which thin film and bulk mechanical properties differ in situ, as described in previous experimental studies.
Having observed the effects of chemical expansion at length scales on the order of hundreds of nanometers, we then turn to characterizing chemical expansion in PCO films using atomic-resolution electron microscopy. By imaging cross-sections of PCO films grown on mechanically-constraining substrates. we characterized chemical expansion in situ, including measuring anisotropic expansion. analyzing the interface structure and dynamics. and characterizing the film composition at different conditions and adjacent to the PCO-YSZ interface and threading defects that propagated from the interface toward the film free surface. From this high resolution characterization, we gained information about the atomic-scale changes that contribute to stress generation, changing elastic properties, and differences between thin film and bulk properties in situ.
Finally. we show that the types of effects observed in the earlier parts of the thesis-including dynamic stress generation in response to electrochemical pumping of ions, volume change as a function of changing oxygen content, and non-stoichiometry dependent mechanical properties- are also present and important in Li storage materials. Specifically. we measured dynamic actuation for spinel-structured LixMn2O4 (LMO) films as a function of Li concentration, predicted computationally that LMO volume increases upon oxygen loss, and showed by nanoindentation that the Young's elastic modulus E, hardness H, and fracture toughness K Ic of LixCoO2 (LCO) all decreased substantially upon Li loss. These results demonstrate the broad applicability of our methods and the chemomechanical coupling effects we have observed even in material systems with differing crystal structure. composition, and mobile ions.
The theme of this thesis is that chemical expansion, a natural consequence of changing defect chemistry in non-stoichiometric oxides, is connected intimately to mechanical variables including stress, strain, and elastic properties. These relationships are particularly important to understand for thin film electrochemical devices that regularly operate in extreme environments or far from equilibrium, where opportunities for strain engineering may enhance performance even as changing environmental conditions may restrict available choices for avoiding mechanical failure. Our results provide a comprehensive study of chemomechanical coupling in a model system, thus providing a framework for modeling similar effects in the diverse set of materials that undergo chemical expansion in operando environments. We also develop experimental tools that can be used to detect these effects in situ , enabling future development of devices based on chemomechanical coupling such as high temperature oxide actuators. Though often framed in the context of energy conversion, our methods and results may be extended to materials for applications that involve dynamic changes in composition or structure during operation, including resistive switches, electrolysis cells, and batteries. (Copies available exclusively from MIT Libraries, libraries.mit.edu/docs - docs mit.edu).
Notlar:
School code: 0753
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Yer Numarası | Demirbaş Numarası | Shelf Location | Lokasyon / Statüsü / İade Tarihi |
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XX(687371.1) | 687371-1001 | Proquest E-Tez Koleksiyonu | Arıyor... |
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