In recent years, both hafnia and zirconia have been looked at closely in the quest for a high permittivity gate dielectric to replace silicon dioxide in advanced metal oxide semiconductor field effect transistors (MOSFET). Hafnium dioxide or HfO2 is chosen for its high dielectric constant (five times that of SiO2) and compatibility with stringent requirements of the Si process. As deposited, thin hafnia films are typically amorphous but turn polycrystalline after a post-deposition anneal. To control the phase composition in hafnia films understanding of structural phase transitions is a first step. In this dissertation using first principles methods we consider three phase transitions of hafnia and zirconia: monoclinic to tetragonal, tetragonal to cubic and amorphous to crystalline. Because the high surface to volume ratio in hafnia films and powders plays an important role in phase transitions, we also study the surface properties of hafnia. We discuss the mechanisms of various phase transitions and theoretically estimate the transition temperatures. We find two types of amorphous hafnia and show that they have different structural and electronic properties. The small energy barrier between the amorphous and crystalline structures is found to cause the low crystallization temperature. Moreover, we calculate work functions and surface energies for hafnia surfaces and show the surface suppression of the phase transitions. / text

La superélasticité (SE) est la capacité d'un alliage à recouvrer sa forme initiale après une importante déformation. Cet effet, dû à une transformation martensitique réversible, est très convoité pour de nombreuses applications biomédicales. Les alliages nickel-titane sont les alliages les plus utilisés dans les applications fonctionnelles nécessitant de la SE. Cependant, l'utilisation de cet alliage dans les dispositifs biomédicaux est controversée par la présence du nickel, considéré comme élément cytotoxique et allergène. De ce fait, ces dernières années, une attention accrue a été portée aux alliages de titane [3-métastable pouvant constituer une alternative pour des applications biomédicales. En effet, ils ont l'avantage d'être élaborés à partir d'éléments biocompatibles et de présenter un comportement SE. L'objectif de cette thèse réside ainsi dans l'étude de la réponse SE des alliages de titane [3-métastable à différentes échelles dans leur forme massive et sous forme de revêtements. Deux alliages [3-métastable ont été élaborés: le Ti-27Nb {% at) et le Ti-24Nb-4Zr-4Sn (% mass). Les propriétés SE de ces alliages à l'état massif ont été caractérisées à l'échelle macroscopique et à l'échelle submicrométrique et comparées à celles du NiTi superélastique et du CP-Ti élasto-plastique. La nanoindentation a d'abord été utilisée pour sonder l'effet SE dans ces alliages massifs à l'échelle locale, et dans un deuxième temps, pour étudier l'effet de l'orientation cristallographique sur la réponse SE et mécanique à l'échelle du grain. Enfin, les propriétés mécaniques et SE de ces alliages sous forme de revêtements ont été évaluées avec ce même procédé de nanoindentation. / The superelasticity is the ability of an alloy to recover its original shape after significant deformation. This effect, due to a reversible stress-induced martensitic transformation, is highly sought after for many biomedical applications. Nickel-titanium alloys that have a very large strain recovery (in bulk and thin film state), are the alloys currently used for functional applications requiring superelasticity. However, the use of this alloy in biomedical devices is controversial by the presence of nickel, considered as allergen and presumed cytotoxic for the body. As a result, in recent years, increased attention has been given to metastable f3 titanium alloys, which may be an alternative for biomedical applications. Indeed, they have the advantage of being elaborated from biocompatible elements and exhibit a superelastic behavior. The objective of this thesis lies in the study of the superelastic response of metastable f3 titanium alloys at different scales in bulk and thin film state. Two metastable f3 titanium alloys were elaborated: Ti-27Nb (at%) alloy and Ti-24Nb-4Zr-8Sn (wt %) alloy. The superelastic properties of these two bulk alloys were characterized at macroscopic and sub-micrometric scale and compared to superelastic NiTi and elastoplastic CP-Ti. The nanoindentation was first used to evaluate the superelastic effect of various bulk titanium alloys at local scale, and in a second time, to study the effect of crystallographic orientations on the superelastic and mechanical responses at the grain scale. Finally, the mechanical and superelastic properties of metastable f3 titanium coatings were evaluated with this same nanoindentation process.

Thesis. 1978. Sc.D.--Massachusetts Institute of Technology. Dept. of Materials Science and Engineering. / MICROFICHE COPY AVAILABLE IN ARCHIVES AND SCIENCE. / Vita. / Includes bibliographical references. / Sc.D.

TiNi films were prepared by co-sputtering TiNi target and a separate Ti target. Crystalline structure and phase transformation behaviors of TiNi films were investigated. Results showed that TiNi films had fine grain size of about 500 nm and fully martensitic structure at room temperature. X-ray photoelectron spectroscopy (XPS) results indicated that there is adherent and natural TiO₂ film, which is beneficial to its corrosion resistance and biocompatibillity. Results from differential scanning calorimeter (DSC), in-situ X-ray diffraction (XRD) and curvature measurement revealed clearly martensitic transformation upon heating and cooling. The TiNi films were further deposited on micromachined silicon cantilever and membrane structures in order to form micro-gripper or microvalve with large deformation due to shape-memory effect. / Singapore-MIT Alliance (SMA)

Ni-rich NiTi shape memory alloys (SMAs) are capable of attaining a wide range of transformation temperatures depending on the heat treatment conditions and superior thermo-mechanical cycling stability, which are desired for repeated solid-state actuation. High Ni-content Ni-rich SMAs have very low transformation temperatures in a solutionized condition due to the high Ni-content of the matrix. Slow cooling (furnacecooling) from solutionizing temperature and additional aging heat treatments result in the formation of Ni-rich precipitates such as Ni4Ti3, Ni3Ti2 and Ni3Ti and increase transformation temperatures above ambient by depleting excess Ni from the matrix. However, the precipitates do not undergo a martensitic phase transformation and they decrease the transformation strain by reducing the volume fraction of the material capable of transforming. Meanwhile, recent preliminary work shows that Ni3Ti precipitates dominate fatigue failure. The objectives of the present study are: (1) to eliminate Ni3Ti but still have Ni4Ti3 precipitates, which are responsible for the dimensional stability and increase transformation temperatures, (2) to investigate the effect of heat treatments on the transformation strain, and (3) to select single variant Ni4Ti3 precipitates through constrained aging for the formation of oriented internal stress and eventually obtain twoway shame memory effect (TWSME) and enhanced dimensional stability. Based on these objectives, the effect of aging heat treatment on transformation temperatures, microstructural evolution, and shape memory behavior were investigated for a Ni52Ti48 shape memory alloy (SMA) by using differential scanning calorimetry (DSC), optical microscopy, scanning electron microscopy (SEM), and thermo-mechanical testing, including isobaric heating-cooling experiments under various stress levels. It was observed that solutionizing at 900 degree C for 24 hours eliminated Ni3Ti type precipitates, but additional aging heat treatments are needed to form Ni4Ti3 precipitates to increase transformation temperatures. Furnace-cooling and additional aging heat treatment results in the multi-stage martensitic transformation due to chemical and stress inhomogeneities in the microstructure. Aging of the controlled furnace-cooled material at 400 degree C for 48 hours resulted in the highest transformation temperatures among all processing conditions investigated due to the combination of Ni3Ti precipitates and 27 percent volume fraction of the Ni4Ti3 precipitates, which led to the depletion of Ni from the transforming matrix. However, since overaging results in losing coherency of the precipitates, dimensional stability during isobaric thermal cycling was negatively impacted.

Thesis (Ph. D.)--Hong Kong University of Science and Technology, 2002. / Includes bibliographical references (leaves 155-160). Also available in electronic version. Access restricted to campus users.

Thesis (M. Phil.)--Hong Kong University of Science and Technology, 2003. / Includes bibliographical references (leaves 94-98). Also available in electronic version. Access restricted to campus users.

No description available.

Thesis (Ph.D.)--Hong Kong University of Science and Technology, 2008. / Includes bibliographical references (p. 125-131).

Thesis (Ph.D)--Mechanical Engineering, Georgia Institute of Technology, 2010. / Committee Chair: David L. McDowell; Committee Member: G. B. Olson; Committee Member: K. A. Gall; Committee Member: Min Zhou; Committee Member: R. W. Neu. Part of the SMARTech Electronic Thesis and Dissertation Collection.