3. Electronic Theses and Dissertations (ETDs) - All submissions
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Item Study of diamond/mullite composites by sol/gel and hot press sintering methods(2011-04-15) Govo, Simbarashe PinielA study has been conducted into the synthesis of 10wt% diamond/ mullite composites through two methods: First through the hot press sintering of alumina and silica in stoichiometric composition for 3:2 mullites (mullite formed in situ) at 1400, 1450 and 1500oC. Second through the sol/ gel process. The sol/ gel method only provided the basis for future development with no further discussion of the results while the hot press sintering method yielded composites with residual cristobalite and corundum phases. Achieved densities of the composites were 93.7, 94.6 and 95.8% of the theoretical density with respect to sintering temperatures of 1400, 1450 and 1500oC for compact samples by the first method. Hardness – measured by Vickers indentation – of the composites decreased with increase in temperature with 15.5 ± 0.33GPa achieved at the lowest sintering temperature investigated. The decrease in hardness was attributed to the structural degradation of diamond to non-diamond carbon forms with increase in temperature as observed from Raman spectra of each of the composites. X-ray traces showed an increase in the mullite content with increase in temperature. The fracture toughness of compacts initially hot press sintered from alumina and silica in stoichiometric composition for 3:2 mullite with no diamond added decreased with increase in sintering temperature with 4.75 ± 0.10MPa·m1/2 achieved at the lowest sintering temperature investigated. Further discussion to the structure and physical properties is presented.Item Cross-section transmission electron microscopy of radiation damage in diamond(2008-03-06T07:29:49Z) Nshingabigwi, Emmanuel KorawingaAbstract Diamond is nowadays regarded as a potential semiconductor material of the future, due to its extreme and unique properties. Some of these properties, in- clude its high hardness, highest breakdown ¯eld, high Debye temperature, high thermal conductivity, high hole and electron mobilities, large bandgap and op- tical transparency, among others. These properties make diamond suitable for high-temperature, high-speed and high-power electronic applicatons, as well as in other applications. However, defects associated with ion implantation have been shown to make it rather di±cult to obtain n-type doping in diamond. As such, an understanding of the nature of defects produced during ion implanta- tion of diamond remains a subject of great importance, if not essential, for the optimization of high-temperature, high-power electronic applications in partic- ular. In this respect, this study investigates the nature of the radiation damage generated within the collision cascades of multi-implantations of carbon ions in high-pressure, high-temperature single-crystal synthetic type Ib diamond, spread over a range of energies (50-150keV) and doses. This is achieved by means of the cold-implantation-rapid-annealing (CIRA) routine, and the anal- ysis of damage caused was done by using cross sectional transmission electron microscopy techniques. More precisely, the modes used to achieve this are the bright ¯eld transmission electron microscopy (BFTEM) coupled with selected area di®raction or SAD. At low dose implantation or at sub-critical implantation doses (2.5x1015 ions/cm2), it was found that the ion-damaged diamond layer consists of some threading dislocations, not homogeneously distributed which propagate from the surface into the ion-damaged diamond. In contrast to the sub-critical implantation doses , it was found that at very high implantation doses (7.0x1015 ions/cm2), i.e., above the critical dose (where diamond transforms to graphite upon annealing), the damaged diamond layer had some unconventional defect features close to the implanted surface.