TitleHigh pressure phase transformations in polycrystalline yttrium oxide
NameDeutsch, Stuart (author), Kear, Bernard H (chair), Tse, Stephen D (internal member), KLEIN, LISA C (internal member), Tsakalakos, Thomas (internal member), Voronov, Oleg A (outside member), Rutgers University, Graduate School - New Brunswick,
SubjectMaterials Science and Engineering,
High pressure (Science)--Research,
High pressure (Technology)
DescriptionNumerous studies have been performed on high-pressure/high temperature phase transitions in rare-earth sesquioxides. Most of these studies were performed using diamond-anvil presses, which limits the size of samples that can be processed. Hence, studies of microstructural and properties changes accompanying phase transitions have been largely neglected. The purpose of this study has been to fill this gap, working with polycrystalline cubic-Y2O3 because of its importance in IR window and dome applications. We selected Diamond Materials Inc. as partner in this investigation, since this company has the expertise to make test pieces under well-controlled HPHT-processing conditions, thus ensuring that the results obtained for one batch of samples to the next are reproducible. This turned out to be crucial, since variations in applied pressure (1.0 to 8.0 GPa range), and holding times (seconds to hours), resulted in significant changes in observed micro/nano-structures. The temperature was fixed at 1000°C in order to limit HPHT-processing variables to pressure and holding time. In view of the results reported here, it now seems clear that extending the investigation to higher temperatures and lower pressures would be productive. The principal accomplishments of this research are as follows: (1) optimization of a reversible-phase transformation process to convert polycrystalline cubic-Y2O3 into the nanocrystalline state, involving a forward-phase transformation from cubic-to-monoclinic (c-to-m) Y2O3 at a high pressure (8.0 GPa) followed by a reverse-phase transformation from monoclinic-to-cubic (m-to-c) Y2O3 at a lower pressure (1.0 GPa); (2) discovery of a transformation-induced crystallization process to convert polycrystalline c-Y2O3 into columnar-grained m-Y2O3, and possibly into single-crystal m-Y2O3 - the driving force is attributed to a pressure-induced phase transformation that occurs at the tips of the growing columnar-grains; (3) formation of a mixed-phase (c-Y2O3/m-Y2O3) nanocomposite due to incomplete reverse transformation from m-to-c Y2O3 - a near 50:50 nanocomposite displays the highest hardness; and (4) insight into infiltration of carbon-containing gases (e.g. CO, CO2), formed via reactions between carbon heater and entrapped gases (e.g. O2, H2O) in the pressure cell, into cracked grain boundaries to form carbon particles/films via a vapor-deposition mechanism, and into uncracked grain boundaries to form carbon-rich species via a boundary-diffusion mechanism.
NoteIncludes bibliographical references
Noteby Stuart Deutsch
CollectionGraduate School - New Brunswick Electronic Theses and Dissertations
Organization NameRutgers, The State University of New Jersey
RightsThe author owns the copyright to this work.