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  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.
31

Thermal Dependence of Shear Banding in Zr70Ni16Cu6Al8 Bulk Metallic Glasses

Brennhaugen, David Dominikus Eide January 2014 (has links)
Zr70Ni16Cu6Al8 bulk metallic glass samples were cast and prepared for tensiletesting. Tensile tests were done at temperatures 77 K, 150 K and 295K, over a range of effective strain rates between 10−5 and 10−1/s in order toassess the dependence of plastic deformation through shear band sliding onthe aforementioned factors.A strong temperature dependence was found, with maximum shear bandsliding distance at 150 K. The sliding distance was also found to increasewith lowered strain rates. The temperature dependence is likely to stemfrom increased heat dissipation suppressing a decrease in viscosity, while thestrain rate dependence stems from a decrease in energy supply rate.Shear bands appeared at higher stresses at lower temperatures, resultingin larger elastic domains and higher ultimate tensile stresses. A mechanismwhere heat dissipation suppresses the formation of shear bands at low stresseswas proposed.v
32

Permeation and Stability Properties of Asymmetric and Disk Shaped Oxygen Permeable Membranes Intended for Syn-Gas Production

Wagner, Nils January 2012 (has links)
In this study, the oxygen permeation and stability properties of disk shaped and asymmetric membranes, both of the composition La0.2Sr0.8Fe0.8Al0.2O3-δ were investigated. Strontium doped Lanthanum ferrite is an oxygen deficient mixed ionic electronic conducting perovskite related material. Co-doping with Aluminum enhances the stability of the system. Received precursor was ball milled and sieved to 250 µm. Phase purity was determined by means of X-ray diffraction. Further on particle size distribution and surface area data were collected by dynamic light scattering and nitrogen absorption (BET).Ceramic disks were fabricated by dry uniaxial dye pressing and sintered at different temperatures. Densities of green bodies were measured by geometric means to be approximately 40% of theor. and final densities were measured using Archimedes method. to be approximately 96% of theor. at a sintering temperature of 1230 °C for 2 h in air.Asymmetric membranes consist of a porous support and a dense functional layer. For preparation of porous supports the powder precursor was mixed with 30 wt-% carbon black and 3 wt-% ethyl cellulose. Supports were calcined at 950 °C and then dip coated with a stabilized suspension of La0.2Sr0.8Fe0.8Al0.2O3-δ in ethanol. After dip coating, membranes were pre sintered at 1100 °C, dip coated again and finally sintered at different temperatures. Visual densities were investigated by scanning electron microscopy. Final sintering parameters were 1230 °C for 2 h respectively. Furthermore, investigations on surface modification were carried out by wet chemical etching using 6M hydrochloric acid, resulting in a partly or complete dissolved functional layer.Oxygen flux of membranes was finally measured in a flux furnace with coupled gas chromatograph.Disk shaped membranes reached oxygen flux values up to 1.77 ml*min-1*cm-2 and asymmetric membranes up to 15.33 ml*min-1*cm-2. After flux measurements were completed, the membranes were checked for degradation by means of XRD and SEM/EDS. Both, disk shaped and asymmetric membranes were overall single phase but showed some morphological degradation.

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