UMD Theses and Dissertations
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Item HIGH PRESSURE REDOX GEOCHEMISTRY OF TUNGSTEN IN METAL-SILICATE SYSTEMS: IMPLICATIONS FOR CORE FORMATION IN THE EARTH(2011) Shofner, Gregory A.; Campbell, Andrew J; Walker, Richard J; Geology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Geochemical models of core formation are commonly based on core and mantle abundances of siderophile elements. Because of the affinity of these elements for metallic phases, they are thought to be highly concentrated in Earth's core. Tungsten is a moderately siderophile element that may provide constraints on the pressure, temperature, composition, and oxygen fugacity conditions, and on the timing of core formation in the Earth. Previous experimental studies suggest that pressure exerts little influence over tungsten metal/silicate partitioning up to 20 gigapascals (GPa). But, core formation models, based on W, predict metal-silicate equilibration pressures outside the available experimental pressure range, thus, requiring extrapolation. Therefore, higher pressure experimental data on tungsten were needed to constrain this important parameter. High pressure melting experiments were conducted to 50 GPa and 4400 K using a diamond anvil cell, and to 26 GPa and 2500 K using a multi-anvil press. Diamond anvil cell samples were sectioned using a focused ion beam instrument. The W-WO2 oxygen fugacity buffer was characterized to high pressure, also using diamond anvil cells and a multi-anvil press, combined with synchrotron x-ray diffraction. Combining the high pressure W-WO2 oxygen fugacity buffer and the database of metal/silicate partitioning data, a new approach was taken to model the Fe-W exchange reaction. Compared to the common linear method of parameterizing metal-silicate partitioning data, this approach captures the non-linear pressure dependence on oxygen fugacity, and allows for modeling of the excess Gibbs energy of mixing based on the activity ratios of Fe, FeO, W, and WO2. Applying this non-linear parameterization to the problem of core formation in the Earth, a pressure-temperature solution of 38 GPa and 3100 K in a peridotite silicate composition for a single-stage, magma ocean core formation model was determined that constrains equilibrium core formation conditions in the Earth. This solution was further constrained by Ni, Co, and Mo parameterizations from the literature. The pressure and temperature conditions of this solution represents a combination of the averaged effects of a deepening magma ocean through time and the "Moon-forming" impact.Item SENSOR BASED ATOMIC LAYER DEPOSITION FOR RAPID PROCESS LEARNING AND ENHANCED MANUFACURABILITY(2006-03-16) Lei, Wei; Rubloff, Gary W; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)In the search for sensor based atomic layer deposition (ALD) process to accelerate process learning and enhance manufacturability, we have explored new reactor designs and applied in-situ process sensing to W and HfO2 ALD processes. A novel wafer scale ALD reactor, which features fast gas switching, good process sensing compatibility and significant similarity to the real manufacturing environment, is constructed. The reactor has a unique movable reactor cap design that allows two possible operation modes: (1) steady-state flow with alternating gas species; or (2) fill-and-pump-out cycling of each gas, accelerating the pump-out by lifting the cap to employ the large chamber volume as ballast. Downstream quadrupole mass spectrometry (QMS) sampling is applied for in-situ process sensing of tungsten ALD process. The QMS reveals essential surface reaction dynamics through real-time signals associated with byproduct generation as well as precursor introduction and depletion for each ALD half cycle, which are then used for process learning and optimization. More subtle interactions such as imperfect surface saturation and reactant dose interaction are also directly observed by QMS, indicating that ALD process is more complicated than the suggested layer-by-layer growth. By integrating in real-time the byproduct QMS signals over each exposure and plotting it against process cycle number, the deposition kinetics on the wafer is directly measured. For continuous ALD runs, the total integrated byproduct QMS signal in each ALD run is also linear to ALD film thickness, and therefore can be used for ALD film thickness metrology. The in-situ process sensing is also applied to HfO2 ALD process that is carried out in a furnace type ALD reactor. Precursor dose end-point control is applied to precisely control the precursor dose in each half cycle. Multiple process sensors, including quartz crystal microbalance (QCM) and QMS are used to provide real time process information. The sensing results confirm the proposed surface reaction path and once again reveal the complexity of ALD processes.