Grain Boundary Induced Compositional Stresses in Non-stoichiometric Oxide Films

Mandowara, Sunil

Full Metadata

Overview

Year:: 2008
Contributor:: Mandowara, Sunil (creator); Sheldon, Brian (director); Chason, Eric (reader); Shenoy, Vivek (reader); Brown University. Division of Engineering. Materials Science Engineering (sponsor)
Genre:: theses
Subject:: Compositional Stresses; Nanocrystalline Ceria; Phase Transformations; Grain boundaries; Phase transformations (Statistical physics)
Extent:: xxiii, 257 p.
DOI: https://doi.org/10.7301/Z0X34VVJ

Files

Description

Abstract:: For non-stoichiometric oxide thin films (e.g. ceria and titania) in constrained geometries the variation in oxygen partial pressures lead to stresses, thus termed compositional stresses. In this study, the compositional stresses were investigated in the nanocrystalline ceria films for temperature, microstructrual and doping effects to better understand stress generation and control mechanism. First, a theoretical model was developed to relate the core-space charge model for defect chemistry to compositional stresses. The ceria films grown on inert substrates by MOCVD (metal organic chemical vapor deposition) were subjected to high temperature redox annealing cycles and the compositional stresses were measured by monitoring substrate curvature through MOSS (Multi-beam optical stress sensor) and oxygen partial pressure by an in situ YSZ sensor. For the low temperature range (<550?), the compositional stress increased with decreasing grain size implying that the stresses are induced at the GB core. The measured apparent enthalpies were lower (0.2-0.5 eV) and space charge potential were higher (~0.83-0.87 V) than generally reported values (~0.3-0.4 V). For very small film thicknesses (hf < 250 nm), the increasing compositional stresses with increasing film thickness suggest that importance of the space charge regions associated with oxygen deficient surfaces and the reduction induced surface reconstructions. For doped films, the compositional stresses measured were significantly lower and apparent enthalpies were higher compare to undoped films, most likely due to extrinsic nature of defect chemistry and dopant vacancy pairing. The compositional stresses increased with decreasing grain size implying that the stresses were induced by GB core in the doped case as well. At higher temperatures (>550?), the GB core may be undergoing phase transformation resulting in reversal of compositional stresses direction. The phase transformation is associated with the GB core as with decreasing grain sizes the magnitude of delta compositional stresses increases and the post annealing XRD runs revealed only pure ceria phase (fluorite).
Notes:: Thesis (Ph.D.) -- Brown University (2009)

Content

Access Conditions

Rights: In Copyright
Restrictions on Use: Collection is open for research.

Citation

Mandowara, Sunil, "Grain Boundary Induced Compositional Stresses in Non-stoichiometric Oxide Films" (2008). Engineering Theses and Dissertations, Materials Science Engineering Theses and Dissertations. Brown Digital Repository. Brown University Library. https://doi.org/10.7301/Z0X34VVJ

Relations

Collections:

Engineering Theses and Dissertations

Theses and Dissertations for the Engineering department.
...
Materials Science Engineering Theses and Dissertations

Theses and Dissertations for the Materials Science Engineering department.
...