Browsing by Subject "TEM"
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Item Impact of Chemical States on the Effective Work Function of Metal Gate and High-kappa Dielectric Materials on Novel Heterostructures(2012-10-19) Coan, MaryAn experimental and theoretical approach is taken to determine the effect of a heterojunction on the effective work function in a metal/high-? gate stack, the characteristics of aqueous hydrochloric acid cleaned (aq-HCl) GaN surface and the interface between GaN and Al2O3, HfO2 and GaON. The investigation of the effect of a heterojunction on the effective work function in a metal/high-? gate stack found that when a Ge/Si heterostructure on silicon is lightly doped and sufficiently thin, the work function can be extracted in a manner similar to that for a simple silicon substrate. Modifications to the terraced oxide structure are proposed to remove oxidation effects of the alternate channel materials. The extracted work function of TiN with various thicknesses on HfSiO is found to be in agreement with that of TiN on a silicon substrate. X-ray and ultraviolet photoelectron spectroscopy are used to observe the interface electronic states at the GaN (0001) and Al2O3, HfO2 and GaON dielectric interfaces. The GaN is cleaned using aqueous HCl prior to thermal oxidation to form GaON and atomic layer deposition of Al2O3 and HfO2. This was followed by a post deposition anneal. The GaN/HfO2 and GaN/Al2O3 interfaces exhibited dipoles of 1.6 eV and 0.4 eV +/- 0.2 eV, respectively. It is determined that the formation of an interfacial layer at the GaN/HfO2 interface is the primary cause of the larger dipole. Due to the knowledge of the formation of an interfacial GaOx or GaON layer during atomic layer deposition of HfO2, a better understanding of the GaN/GaON interface is needed. To accomplish this task, the interface electronic states at the GaN(0001) and GaON interface are observed using X-ray and ultraviolet photoelectron spectroscopy (XPS and UPS). XPS and UPS analysis of the GaN/GaON interface resulted in the calculation of a -2.7 eV +/- 0.2 eV dipole assuming that the core level shifts are only representative of the GaN band bending at the interface. If it is assumed that the core level shifts are only due to the oxidation of GaN, then the exhibited dipole at the GaN/GaON interface is -1.8 eV +/- 0.2 eV. Results indicate that the observed dipole is primarily caused by the polarization of the GaN. A theoretical approach is taken to provide a more complete understanding of the underlying formation mechanisms of a GaON interfacial layer during atomic layer deposition of HfO2. First, density functional theory is used to calculate the interactions of oxygen and water with the Ga-face of GaN clusters. The GaN clusters could be used as testbeds for the actual Ga-face on GaN crystals of importance in electronics. The results reveal that the local spin plays an important role in these interactions. It is found that the most stable interactions of O2 and the GaN clusters results in the complete dissociation of the O2 molecule to form two Ga-O-Ga bonds, while the most stable interactions between a H2O molecule and the GaN clusters are the complete dissociation of one of the O-H bonds to form a Ga-O-H bond and a Ga-H bond. Second, density functional theory is used to calculate the interaction of the reactants used to deposit HfO2 and Al2O3 during atomic layer deposition with hydrolyzed Ga-face GaN clusters. The results suggest that while further research is needed in this area to grasp a better understanding of the interactions of Trimethylaluminum (TMA) or Tertrakis(EthylMethylAmino)Hafnium (TEMAH) with hydrolyzed GaN clusters, it is found that a Ga-N(CH3)(CH2CH3) bond can form during the deposition of HfO2 using ALD and TEMAH as the reactant without breaking the Hf-N bond. The formation of a Ga-N(CH3)(CH2CH3) bond is significant because with the introduction of water into the system, the methyl and ethylmethyl groups may react to form a Ga-N-O bond which is believed to be the interfacial oxide found during deposition of HfO2 using ALD on GaN. No Ga-C bond structure formed in any fully optimized stable structure when analyzing the interaction of TMA with hydrolyzed GaN.Item Methods for Investigating Gas Bubble Formation in Uranium-Zirconium Alloys(2013-05-06) Mews, Kathryn Ann WrightUranium-zirconium alloy nuclear fuels have many advantages as compared with ceramic fuels, especially for fast reactor systems. However, metallic fuels aren?t currently used in commercial power production due in part to issues with fuel swelling during irradiation. A major contributor to this expansion issue, the formation of fission gasses into bubbles, is examined here. Methods to evaluate evolution of fission gas bubbles within a U-Zr alloy are discussed and refined. Specifically, transmission electron microscopy (TEM) for viewing bubbles within the interior of the alloy is investigated. One constraint on the use of the TEM is the lack of literature pertaining to what thinning techniques are successful for U-10Zr alloy. Both initial and final thinning techniques were investigated. After thinning was complete, the specimens were viewed via TEM to determine their suitability. In addition, samples of U-Zr alloy were irradiated with gas atoms in an accelerator to simulate bubble initiation and formation and viewed via TEM. Only preliminary investigations were completed. Evaluated electrolyte solutions included one part phosphoric acid to two parts sulfuric acid and two parts water (A), one part phosphoric acid to one part ethanol and one part glycerol (B), one part hydrochloric acid to one part water (C), two parts methanol to 13 parts phosphoric acid (D), and one part perchloric acid to nine parts acetic acid (G). Positive responses were received from the solutions A, C, and G which generated electron transparent areas with few to no process induced artifacts. TEM trials with each electrolyte were performed on un-irradiated U-10Zr alloy. These trials indicated that the polishing methods would work but are not yet optimal. In addition, U-10Zr alloy was irradiated and viewed in the TEM where those polished with electrolyte A consistently included the presence of large circular features that could indicate bubbles or voids. The foundation was laid for further study to be done on this topic through the use of electropolishing solutions as final thinning techniques. Additional work that is recommended includes: electropolishing parameter refinement; implantation of heavier gas atoms or at elevated temperatures; annealing of the implanted alloy; and computer modeling of resulting first principles phenomena.Item Solid State Phase Transformations in Uranium-Zirconium Alloys(2013-08-06) Irukuvarghula, SandeepUranium-10wt% zirconium (U-10Zr) alloy nuclear fuels have been used for decades and new variations are under consideration ranging from U-5Zr to U-50Zr. As a precursor to understanding the fission gas behavior in U-Zr alloys using ion implantation, a basic study on the U-Zr metallurgy was completed using EPMA, DSC, XRD, Optical microscopy, and TEM with a focus on solid state phase transformations in alloys containing 2, 5, 10, 20, 30, and 50wt% zirconium. Alloys were cast by crucible melting using high temperature furnace under argon atmosphere in yttrium oxide crucibles and various thermal profiles were used to study phase transformations in these alloys. Using TEM, XRD, and DSC data, it was ascertained that athermal-?, along with martensitic ?1, formed in all alloys quenched from ? phase. XRD could detect the presence of athermal-? only in U-20, 30 and 50wt%Zr alloys. BSE images for as-cast alloys of 2, 5, 10, 20, and 30wt%Zr had lamellar microstructure with lamellae rich in zirconium. All alloy samples clearly showed a heating transformation pertaining to ? ? ? in DSC data while XRD could only confirm the presence of ? phase in U-20, 30, and 50wt%Zr alloys. An explanation is offered for the absence of ? phase peaks in uranium-rich alloys based on its formation mechanism. Alloy samples of U-2, 5, and 10wt%Zr were step-cooled from ? phase by annealing in the (? + ?) phase field before cooling to room temperature revealed broad peaks for ? phase indicating incomplete collapse of {111}? planes. Both as cast and ?- quenched alloys were annealed at 600degreeC, in the (? + ?) phase field for 1, 3, 7, and 30 days. Microstructures of the samples in both cases contained uranium-rich matrix and zirconium-rich precipitates and WDS analyses were consistent with their being ?-U and ? phase respectively. However, XRD data for annealed alloys never showed peaks for ? phase even though it?s area fraction was within the detection limits. Moreover, the peaks which were present in U-20wt%Zr vanished after annealing for 7 days. Based on the data obtained, it is suggested that it is more appropriate to consider the presence of metastable diffusional-? instead of a stable ? in the as-cast alloys and that it is not stable at 600degreeC.Item The effect of strain and path change on the mechanical properties and microstructural evolution of ultrafine grained interstitial free steel during equal channel angular extrusion (ECAE)(Texas A&M University, 2007-04-25) Sutter, Steven GeorgeThe objectives of this study were to examine the effect of strain and path change on the microstructural evolution of ultrafine grained interstitial free (IF) steel during equal channel angular extrusion (ECAE); to determine the mechanical properties; to observe the resulting texture; and to perform optical and electron microscopy of the resulting material. The effects of different routes of extrusion (A, B, C, C' and E), heat treatment and plastic strains from 1.15 to 18.4 were examined. Monotonous tensile testing was used to determine mechanical behavior of processed materials. X-ray diffraction and TEM analyses were performed to evaluate the effect of processing on texture and grain morphology. Hardness measurements were performed to determine recrystallization behavior of the processed material. Optical microscopy was conducted on heat treated samples to determine their grain size and refinement. Monotonous tensile testing of processed materials showed that there was significant strengthening after the first extrusion. Further processing resulted in increasing values of yield strength and ultimate tensile strength, with ductility at failure varying depending upon which processing route was used. The best tensile strength results were obtained after processing Routes 8C' and 16E, due to the significant grain refinement these routes produced. X-ray diffraction revealed increases in strength of preferred texture along the directions [111] and [001], perpendicular to the transverse plane, for all specimens that were processed using ECAE. TEM observations showed a consistent refinement of grain size as the amount of processing increased, especially within Routes C' and E. Hardness measurements of heat treated specimens showed that the onset of recrystallization occurred at approximately the same temperature of recrystallization as that of pure iron, 450????C. The recrystallization curves for all samples showed that grain growth begins at a temperature of around 700????C. The low carbon content of IF steel made optical microscopy challenging. The grain size of annealed materials becomes finer and more uniform, ranging between 60 and 90 ????m2, at high strain levels under Routes C' and E, due to the many potential nucleation sites developed in highly worked material.Item The nanoporous morphology of photopolymerized crosslinked polyacrylamide hydrogels(2009-05-15) Wang, JianNanoporous polymer hydrogels offer a desirable combination of mechanical, optical, and transport characteristics that have placed them at the core of a variety of biomedical technologies including engineered tissue scaffolds, substrates for controlled release of pharmaceutical compounds, and sieving matrices for electrophoretic separation of DNA and proteins. Ultimately, we would like to obtain a detailed picture of the nanoscale pore morphology and understand how it can be manipulated so that we can rationally identify gel formulations best suited for a specific application. But this goal has proven elusive because the most fundamental descriptors of the pore network architecture (e.g., the average pore size and its polydispersity) are particularly difficult to measure in polymer hydrogels. Here we introduce an approach that enables both the mean pore size and the pore size distribution to be quantitatively determined without prior knowledge of any physical material parameters A novel technique to prepare TEM samples was developed so that the nanoscale hydrogel pore size, pore shape and distribution are clearly visualized and quantitatively studied for the first time. The pore sizes of the hydrogel are also estimated with rheology. A new fixture is used in the rheometer and the whole polymerization process can be directly studied using an in-situ rheology experiment. A series of thermoporometry experiments are also conducted, and suitable methods and equations to study hydrogel pore size and distribution are chosen. The pore size derived from TEM, rheology, DSC is compared and their values are self-consistent. These techniques help us understand how the nanoporous morphology of crosslinked polyacrylamide hydrogels is influenced by their chemical composition and polymerization conditions. It is interesting to find hydrogels with similar pore size but different distribution. For two hydrogels with similar pore size, the broader the distribution, the faster the release rate and the higher the accumulated release percentage. So we can control the release of trapped molecules by simply varying the hydrogel pore size distribution. This discovery would have a very promising potential in the application of pharmaceuticals.