The thermal cycling of cadmium and dilute Cd-Pb alloys was studied by measuring the length change of a specimen as a function of the number of cycles a specimen had undergone. The majority of the experiments were performed using material which had been swaged to 90% reduction in area followed by various amounts of drawing reduction, and which were cycled from 0°C to -190°C. Other areas of interest were the effect of temperature range and grain size on the growth rate, and the effect of cycling on the microstructure and on the axial thermal expansion coefficient. It was found that swaged-and-drawn material cycled from 0 to -190°C, exhibited a three-stage growth curve. The growth rates in Stages 2 and 3 were constant; Stage 1 was ill-defined and was not investigated to any extent. The transition from stage to stage was abrupt and was accompanied by a decrease in growth rate, a lower incidence of non-basal slip and a change in the expansion coefficient. Both the Stage 2 and the Stage 3 growth rates showed a (grain size)¯ ½ dependence and reacted in approximately the same way to alloying in that maxima in both growth rates were indicated for alloys containing between 10 and 50 ppm Pb. The results were interpreted in terms of an idealized model of a textured specimen. The model consisted of a bicrystal representing all grains possessing a given orientation, surrounded by a matrix representing all other grains in the specimen. Stresses are set up in the specimen since the expansion of the textured grains and the matrix grains differ in a given reference direction. These expansion stresses are then relaxed at the upper temperature by creep processes. Stage 3 was identified with the overall cycling mechanism. Stage 2 was considered to result from the interaction of the swaged and the drawn portions of the swaged-and-drawn texture. Stage 1 was believed to result from boundary stresses arising from the differential expansion of adjacent grains. Some experiments were carried out using rolled, drawn and swaged material. The results of these experiments as well as those results obtained by changing the temperature range were used primarily for purposes of comparison to the results obtained from cycling swaged-and-drawn specimens from 0 to -190° C. / Applied Science, Faculty of / Materials Engineering, Department of / Graduate

Semi-insulating, powder samples of Cadmium Telluride (CdTe) have been studied using ������In Time Differential Perturbed Angular Correlation (PAC) Spectroscopy. The samples have been lightly doped (~10���� cm�����) with ������In atoms, which occupy well-defined metal (Cd) lattice sites and act as probes of the local environment. These substitutional donors form a single defect complex in CdTe. This complex has been identified and characterized as a function of temperature. Those indium probes that are not complexed occupy metal lattice sites with no defect in the local vicinity. Samples containing metal vacancy concentrations as large as 500 ppm have been prepared by a high temperature anneal and quench. The defect complex involves the trapping of a cadmium metal vacancy bound to the indium probe. The electric field gradient (EFG) experienced by probe atoms has a coupling constant of V[subscript Q]=61.5(5) MHz and is not axially symmetric, with the asymmetry parameter given by ��=0.16(4). It is believed that this asymmetry results from a relaxation of the chalcogen (Te) atoms adjacent to the metal vacancy, with the tellurium atom shared by the probe atom and the vacancy providing the dominant contribution. The fraction of complexed probe atoms increases as the sample temperature is decreased, and is still increasing at room temperature. Complexed fractions are reproducible on cycling within the temperature range 40 to 200��C. The binding energy of the complex has been measured to be 0.15(2) eV and is independent of metal vacancy concentration, which varies and is dependent on the details of the quench. In rapidly cooled samples, a non-equilibrium number of these defect complexes is observed. This state equilibrates with a time constant of 45(5) hours at 15��C, implying that at least one of the two constituents involved in the complex has a significant diffusion rate at this temperature. Under the assumption that vacancy diffusion mechanisms dominate at this temperature, it is found that the cadmium vacancies overcome an energy barrier of 0.9(1) eV with a jump time of 20(2) minutes in CdTe at 15��C. / Graduation date: 2002

No description available.

No description available.

We report here a study of a specific doublet of de Haas-van Alphen frequencies in pure Mg and very dilute Mg(Cd) alloys with the magnetic field aligned with the c-axis. The work involved three stages. First, the use of extremely strain-free crystals, temperatures down to 40 millidegree Kelvin, large amplitude modulation, and the fast Fourier transform allowed the components of this doublet to be well resolved. This resolution allowed measurement of the changes in the cross-sectional area as a function of magnetic field orientation to verify the assignment of this doublet to the cap and monster arm junction at the top of the Brillouin zone. Third, with the magnetic field aligned with the c-axis, the splitting of this doublet offered a direct and sensitive indication of any symmetry breaking changes in the 0001 Fourier component of the ionic lattice potential in Mg upon the introduction of Cd. C. B. Friedberg's analysis of his electron interference lineshape data from the quantum interferometer in Mg had indicated that the energy of this band gap should increase by 40% with the introduction of 15 ppm Cd. Our data indicate that any change in the energy of the band gap must be at least three orders of magnitude smaller than that indicated by Friedberg. Our data are, in fact, consistent with there being no changes in the electronic band structure or the Fermi surface of Mg(Cd) alloys (with up to 0.02% (At) Cd), from that of pure Mg.