No description available.

No description available.

No description available.

This thesis contains three major areas of work. Chapter 1 describes the various film deposition techniques available for the growth of thin films. These different growth methods, substrate preparation and vacuum techniques, are discussed in the context of the effect of the growth process and deposition environment on film structure and quality. Chapter 2 describes the two film growth chambers. Chapter 3 describes the origin of giant magnetoresistance (GMR) in magnetic multilayered structures, and investigates the magnetoresistive properties of ion-beam sputtered CoCu multilayered thin films. Different film structures and deposition conditions have been investigated in order to maximise the size of negative magnetoresistance seen. Chapter 4 investigates the magnetic and giant magnetoresistive properties of sputtered CoCu and CoAg heterogenous alloys. The magnitude of the GMR effect is found to depend on the Co concentration and Co particle size. Chapter 5 describes an investigation of molecular beam epitaxy (m.b.e.) grown Dy/Y multilayer structures. The temperature dependences of helical magnetic structure in the Dy layers is investigated in two samples of different multilayer structures. Chapter 6 describes an acoustic interferometer which has been used to measure velocity and attenuation changes in an ultrasonic signal at frequencies of order IGHz. Also described is the production of the thin film piezoelectric transducers used in this technique, which are grown directly onto a single crystal of the material under investigation. The acoustic interferometer is used to investigate the dynamics of of a relaxation process between two time-reversed antiferromagnetic states in DyAlG, and a model for domain growth in this system is postulated. Chapter 7 investigates the phase diagram of DyPO₄ using the acoustic interferometry technique. The observations of hysteresis effects are used to define a mixed phase region below the tricritical point.

The effects of systematic variations in the size and volume fraction of reinforcing phase on the mechanical properties of and fracture processes in silicon carbide particlereinforced aluminium matrix composites have been studied. Tensile tests to failure have been performed to determine the mechanical properties of the composites. A simple model has been proposed for this behaviour. The micromechanisms of fracture have been investigated by a combination of fractographic and dynamic techniques. Matched fracture halves have been obtained from the composites and the fracture processes elucidated. Fracture proceeded by a ductile void nucleation, growth and coalescence mechanism. Void nucleation occurred at the reinforcing phase, with a change in nucleation mechanism on varying the micrstructural parameters. A simple critical stress criterion has been proposed for the nucleation process. Support for this proposal has been obtained by the study of sections through the failed tensile specimens. In situ scanning electron microscopy fracture studies have been performed. These revealed void nucleation before the onset of macroscopic cracking. Crack propagation has been shown to occur by the concurrent formation of microcracks ahead of the crack tip and failure of the joining matrix ligaments. The magnitude of matrix deformation has been shown to determine the extent of microcracking. Acoustic emissions have been monitored during tensile straining. Void nucleation events have been recorded from the onset of plastic deformation and continuing throughout the plastic régime until final failure. The suppression of void coalescence by the constaint imposed on matrix flow by rigidly-bonded interfaces has been proposed to account for the extended void growth in materials containing fractured particles. The importance of the local values of the microstructural parameters on the far-field strain at nucleation has been shown.

No description available.

In order to clarify the role that water plays in the photocatalytic process, changes in the IR and Raman spectra of P25 TiO₂ thin films were observed upon exposure to liquid water. Further investigation of these spectral changes via dehydration of thin films under nitrogen and oxygen of different humidities led to the observation of spectroscopic features that have been assigned to localised surface phonon modes. When the effect of UV irradiation on these features was investigated, a broad IR absorption due to transitions of electrons in shallow traps was detected under dry nitrogen but not under dry oxygen. Further investigation of the photocatalytic properties of P25 TiO₂ showed a complete removal of a stearic acid film. The final products have been tentatively assigned to a mixture of short chain carbonyl species and adsorbed carbonates as well as carbon dioxide and water. The IR spectrum of the fuel cell membrane material Nafion is complex and literature data varies in some of the assignments. The compound perfluoro(2-ethoxyethane)sulfonic acid was used as a model compound for the Nafion side chain resulting in a clearer assignment of the Nafion IR spectrum. In light of these new assignments changes induced in the region 1100-1300 cm⁻� by variation in humidity and ion exchange have been shown to be mainly the result of changes in the sulfonate asymmetric stretching modes. By flowing a series of solutions containing tetramethylammonium ions and perchlorate ions the surface charge characteristics of a Pt black film were determined in the pH range 2-12. There proved to be a weak positive charge below pH 4 and a weak negative charge above pH 9. Between these points there appeared to be no overall charge on the surface. When perfluoro(2-ethoxyethane)sulfonic acid was adsorbed to a Pt black film changes in its IR spectrum indicated a strong binding via interactions between the sulfonate groups and the Pt surface. The nature of the adsorption of Nafion was less clear cut and, whilst adsorption is strong, it seems possible that hydrophobic interactions between the Nafion backbone and the surface are involved.

Thesis (MEng)--University of South Australia, 1996

A repetitive low-pressure die casting technique has been developed for casting high quality Mg6SCU2SY10 bulk metallic glass (BMG) samples. Using these as-cast samples, the thermal and mechanical behaviour of the BMG in the supercooled liquid (SCL) region was investigated principally by uniaxial tensile testing, differential scanning calorimetry (DSC), X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Using the die casting facility, charge temperature, injection pressure and injection velocity were found to be important parameters for controlling the length, porosity and degree of crystallinity in the as-cast samples. A processing map was generated which showed that a melt temperature and casting pressure in the range 560-580°C and 0.4-0.5 bar, respectively, generated the highest quality samples. The static crystallization behaviour of the as-cast BMG was investigated in detail. It was found that the kinetics of both anisothermal and isothermal crystallization were adequately represented by a number of transformation models. Using the Johnson-Mehl-Avrami-Kolmogorov (JMAK) relation in conjunction with isothermal DSC, the Avrami exponent was found to vary from 2.2 to 2.5 with increasing annealing temperature which implies that, at high annealing temperatures, nucleation occurs at a constant rate accompanied by diffusion-controlled growth of spherical grains. A comparable Avrami exponent of 2.34 was also calculated by anisothermal DSC using the Ozawa method. The tensile flow behaviour of the BMG was investigated over a range of strain rates (10.3to 10.1S•1) and deformation temperatures (150 to 170 QC) in the SCL region using standardized tensile test samples. It was found that, the flow stress increased rapidly to a maximum value followed by a decrease to a very low steady-state value. In the SCL region, the relationship between peak flow stress, strain rate and absolute deformation temperature was described adequately by the classic Sellars-Tegart constitutive relationship. There was also a good correlation between the Zener-Hollomon parameter, Z, and the flow characteristics of the BMG such as the transition from Newtonian to non-Newtonian flow at Z>1 031 S•1 and optimum superplasticity for Z-values in the range 5 x 1030 to 5 x 1031 S•1 where tensile elongations in excess of 1400% were achieved.

Thesis (M.S.)--Rochester Institute of Technology, 1989. / Includes bibliographical references (leaves 74-76).