published_or_final_version / abstract / Physics / Master / Master of Philosophy

published_or_final_version / abstract / Physics / Master / Master of Philosophy

published_or_final_version / Physics / Master / Master of Philosophy

No description available.

No description available.

Cu2ZnSnS4 (CZTS) is a promising absorber material for thin-film solar cells since it contains no rare or toxic elements, has a high absorption coefficient and a near ideal bandgap energy. It does, however, present some challenges due to the limited single-phase region of the desired kesterite phase and its instability towards decomposition. Sputtering of CZTS from quaternary, compound targets using RF magnetron sputtering is known. In this thesis work CZTS absorbers were made using pulsed DC magnetron sputtering on stainless steel substrates. The effects of varying substrate temperature and adding seed layers to promote grain growth were investigated, as well as the effects of a rapid thermal anneal in a S-rich atmosphere. Film compositions determined by X-ray fluorescence were found to be inside or close to the kesterite single-phase region in the phase diagram, but were generally too Cu-rich and Zn-poor to yield good results. The kesterite phase was confirmed with X-ray crystallography and Raman spectroscopy, indicating that it is possible to sputter CZTS from a single target with a high deposition rate. It was found that Cu2S seed layers could induce a significant increase in grain size, and preliminary experiments showed no evidence of the seed layer remaining after deposition of the absorber. Higher substrate temperatures also lead to increased grain size, but excessive heating caused the decomposition of the CZTS. Annealing induced grain growth, relaxed internal stress in the material and improved the electrical properties of the CZTS films, primarily by the removal of shunts.

Evidence is presented that apparent silene products obtained from the metalation of cyclopentadienyldimethyl - chlorosilane either with tert-butyl1ithium or with methylenetriphenylphosphorane actually arise from the metalated starting material, a silenoid, rather than from a silafulvene intermediate. Trimethylmethoxysi1ane is shown to be an effective trap for dimethylsilafulvene. A new dimethylsilafulvene precursor, bis(dimethylmethoxysi1yl) cyclopentadiene, which gives high yields of dimethyldimethoxysi1ane and the silafulvene at temperatures as low as 240°C is reported.

Signal Amplification by Reversible Exchange (SABRE) is a hyperpolarization technique that utilizes parahydrogen for the NMR signal enhancement of nuclear spins. SABRE is related to Parahydrogen Induced Polarization (PHIP), another means of hyperpolarization using parahydrogen; PHIP achieves hyperpolarization via chemical reduction. Although PHIP and SABRE share many similarities in experimentation, PHIP ultimately requires the presence of an unsaturated chemical bond as well as pairwise-addition of parahydrogen. No permanent chemical change occurs during SABRE, and instead may be considered as a merely physical exchange between molecules with sites on a catalyst. PHIP and SABRE may be compared to Dynamic Nuclear Polarization (DNP), arguably the most well-known and researched method for hyperpolarization; despite all that has been achieved with DNP, PHIP and SABRE offer vastly more-rapid, less-expensive, and more-simplified approaches for achieving hyperpolarization. The focus of this work is experimentation with SABRE processes and methods designed to overcome certain experimental challenges associated with this technique.

xvi, 145 p. : ill. (some col.) / The structures of several compounds in the [(PbSe)0.99]m[WSe2]n, [(PbSe)1.00]1[MoSe2]1, and [(SnSe)1.03]1[MoSe2]1 systems were determined using x-ray data. The structural determination using Rietveld methods was complicated by the strong preferred orientation of the samples, which resulted in x-ray diffraction scans with either 00l or hk0 reflections depending on the orientation of the sample in the diffractometer. Rietveld refinements of the [(PbSe)0.99]1[WSe2]1, [(PbSe)1.00]1[MoSe2]1, and [(SnSe)1.03]1[MoSe2]1 samples were compared to single crystal sample refinement results for [(MX)1+d]1[TX2]1, where M is a metal, T is a transition, X is a chalcogen, and d is the misfit parameter. The structural refinement yielded rock salt layer puckering values of 25 pm, 23 pm, and 36 pm for [(PbSe)0.99]1[WSe2]1, [(PbSe)1.00]1[MoSe2]1, and [(SnSe)1.03]1[MoSe2]1, respectively, which are all within the established literature range of 20 pm to 60 pm. The refinement of the hk0 reflections confirmed that the in plane structures were consistent with the dichalcogenide (P63mmc) and rock salt (Fm3m) structure types. Structures for the [(PbSe)0.99]m[WSe2]m isomer series where m = 1 to 5 were determined, and a systematic trend in structure as a function of the thickness of the constituent layers was discovered. The structure of the rock salt constituent was found to distort into pairs, forming alternating long and short distances along the c axis. This distortion decreases as the number of rock salt planes increases from 4 to 6 to 8 and is either absent or nearly so in compounds with a larger number of rock salt planes. The puckering distortion at the interface between the rock salt and the dichalcogenide is also observed in the inner rock salt layers but decreases in magnitude moving away from the rock salt – dichalcogenide interface. Structures of [(PbSe)0.99]m[WSe2]n where m = 1 or 2 and n = 1 or 2 were also determined. The degree of structural distortion is a function of the ratio of rock salt to dichalcogenide layers. This dissertation includes unpublished co-authored material. / Committee in charge: Dr. Thomas R. Dyke, Ph.D., Chairperson; Dr. David C. Johnson, Ph.D., Advisor; Dr. Catherine J. Page, Ph.D., Member; Dr. Andrew H. Marcus, Ph.D., Member; Dr. John L. Hardwick, Ph.D., Member; Dr. Richard Taylor, Ph.D., Outside Member

Properties of cast aluminum components can be improved by strategically placing ferrous inserts to locally improve properties such as wear resistance and stiffness. A cost-effective production method is to cast-in the insert using the solidification of the molten aluminum as a joining method. Metallurgically bonding between the metals could potentially improve both load and heat transfer across the interface. The metallurgical bond between the steel and the aluminum has to be strong enough to withstand stresses related to solidification, residual stresses, thermal expansion stresses, and all other stresses coupled with the use of the component. Formation of a continuous defect free bond is inhibited by the wetting behavior of aluminum and is governed by a diffusion process which requires both energy and time. Due to the diffusional nature of the bond growth in combination with post manufacturing heat treatments defects such as Kirkendall voids can form. The effect of aluminum alloying elements during liquid-solid bond formation in regards to microstructural changes and growth kinetics has been described. A timeframe for defect formation during heat treatments as well as microstructural changes has been established. The effect of low melting point coatings (zinc and tin) on the nucleation of the metallurgical bond has been studied as well the use of a titanium coating for microstructural modification. A set of guidelines for successful metallurgical bonding during multi- material metal casting has also been constructed.