July 2012 - Msepostdoc-list

Seminar Reminder - N. Anderson, Tue. 7/24 9:00 AM, ARMS 1028
by Stacey, Lisa A 20 Jul '12

20 Jul '12

MATERIALS SCIENCE AND ENGINEERING SEMINAR Structural, Thermodynamic, Electronic, and Magnetic Characterization of Point Defects in Amorphous Silica by Nathan L. Anderson Ph.D. Final Exam Advisor Professor A. Strachan ABSTRACT A completely first-principles procedure for the creation of experimentally validated amorphous silicon dioxide structures via a combination of molecular dynamics and density functional theory is presented. Point defects are analyzed within a statistical ensemble of these structures and overcoordinated silicon and oxygen defects are found to have similar formation energies to undercoordinated silicon atoms and oxygen vacancies. The formation of E' centers is found to occur in the absence of oxygen vacancies, and a single oxygen vacancy is found to lead to two isolated E' center precursors. Density functional techniques that properly account for the electrostatics in the presence of periodic boundary conditions are then used to add and remove electrons from each defect and the trapping level distributions are identified. These distributions are the result of the inherent local atomic variability in the amorphous network. The distribution energies are in good agreement with trap spectroscopy experiments where defect contributions are experimentally indistinguishable. This ability to distinguish defect contributions is used to provide a physical explanation of the atomic relaxations which occur upon electron or hole capture. The paramagnetic [cid:image003.png@01CD669A.0D975270] and [cid:image005.png@01CD669A.0D975270] defects are shown to exist in the neutral charge state and are capable of trapping both electrons and holes. Statistical support for the oxygen vacancy originated dimerized model of the positively charged [cid:image007.png@01CD669A.0D975270] defect is demonstrated. An overlap of distributions for different defects is also found suggesting the existence of less known trapping mechanisms involving positively charged overcoordinated oxygen defects and overcoordinated silicon floating bond defects. Further, the uncertainty from the model form that results from exchange-correlation functional choice in density functional theory is quantified and found to be much less than the inherent atomic variability in the amorphous network. Extending these amorphous structure prediction and defect analysis methods to silicon/silica interfaces and silicon/germanium transistors is also discussed. Date: Tuesday, July 24, 2012 Time: 9:00 AM Place: ARMS 1028 Lisa Stacey Secretary/Development Assistant Purdue University School of Materials Engineering 765/494-4100

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Seminar Notice - N. Anderson, Tue. 7/24/12, 9:00 am, ARMS Rm 1028
by Stacey, Lisa A 17 Jul '12

17 Jul '12

MATERIALS SCIENCE AND ENGINEERING SEMINAR Structural, Thermodynamic, Electronic, and Magnetic Characterization of Point Defects in Amorphous Silica by Nathan L. Anderson Ph.D. Final Exam Advisor Professor A. Strachan ABSTRACT A completely first-principles procedure for the creation of experimentally validated amorphous silicon dioxide structures via a combination of molecular dynamics and density functional theory is presented. Point defects are analyzed within a statistical ensemble of these structures and overcoordinated silicon and oxygen defects are found to have similar formation energies to undercoordinated silicon atoms and oxygen vacancies. The formation of E' centers is found to occur in the absence of oxygen vacancies, and a single oxygen vacancy is found to lead to two isolated E' center precursors. Density functional techniques that properly account for the electrostatics in the presence of periodic boundary conditions are then used to add and remove electrons from each defect and the trapping level distributions are identified. These distributions are the result of the inherent local atomic variability in the amorphous network. The distribution energies are in good agreement with trap spectroscopy experiments where defect contributions are experimentally indistinguishable. This ability to distinguish defect contributions is used to provide a physical explanation of the atomic relaxations which occur upon electron or hole capture. The paramagnetic [cid:image003.png@01CD640D.913478E0] and [cid:image005.png@01CD640D.913478E0] defects are shown to exist in the neutral charge state and are capable of trapping both electrons and holes. Statistical support for the oxygen vacancy originated dimerized model of the positively charged [cid:image007.png@01CD640D.913478E0] defect is demonstrated. An overlap of distributions for different defects is also found suggesting the existence of less known trapping mechanisms involving positively charged overcoordinated oxygen defects and overcoordinated silicon floating bond defects. Further, the uncertainty from the model form that results from exchange-correlation functional choice in density functional theory is quantified and found to be much less than the inherent atomic variability in the amorphous network. Extending these amorphous structure prediction and defect analysis methods to silicon/silica interfaces and silicon/germanium transistors is also discussed. Date: Tuesday, July 24, 2012 Time: 9:00 AM Place: ARMS 1028 Lisa Stacey Secretary/Development Assistant Purdue University School of Materials Engineering 765/494-4100

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Seminar Reminder - Efe, Tue. 7/17, 9:30 AM, ARMS Rm 3115
by Stacey, Lisa A 17 Jul '12

17 Jul '12

MATERIALS SCIENCE AND ENGINEERING SEMINAR Extrusion Machining: A Thermo-Mechanical Process for Producing Strips of Alloys Having Limited Workability by Mert Efe Ph.D. Final Exam Advisor Professor K. Trumble ABSTRACT Deformation processing of alloys with limited workability is one of the principal technological challenges for the metal manufacturing industry. Traditional processes (rolling, extrusion) are usually performed at high temperatures (0.5 Tm) for increased plasticity and crack-free products, which makes them costly, energy intensive and raw material inefficient. In this thesis, extrusion machining (EM) a shear-type deformation process is applied to alloys with limited workability for sheet and foil production from bulk forms in a single step. EM is shown to have a narrowly confined and controllable deformation zone, with attributes ranging from conventional deformation processing to severe plastic deformation. Controllable deformation parameters include strain, strain rate, hydrostatic pressure, temperature and deformation path. These attributes are highlighted in deformation processing of two classes of limited workability alloys: 1) Alloys with intrinsic susceptibility to cracking (and segmentation) due to flow localization and shear band formation and 2) Alloys with extrinsic susceptibility to cracking due to macroscopic defects such as porosity, casting defects and weak second phases. Magnesium AZ31 alloy is selected as a model material for the first class and sintered copper and cast brass alloys are selected for the second class. Noteworthy features of the process are suppression of segmentation by combinations of high hydrostatic pressures (p/2k = 1.7) and deformation temperatures (T > 0.5 Tm) realized by in-situ plastic heating, realization of a range of strains [cid:image006.png@01CD63F8.F1CFA950] and deformation rates [cid:image007.png@01CD63F8.F1CFA950] , engineering of microstructures ranging from conventional to ultrafine grained, and creation of sheet from the bulk in a single step of deformation without pre-heating. The thermo-mechanical conditions and microstructural processes that are responsible for continuous and sound sheet products are established. Guidelines for scalability and cost-effectiveness of EM for commercial production are analyzed and discussed. Date: Tuesday, July 17, 2012 Time: 9:30 AM Place: ARMS 3115 Lisa Stacey Secretary/Development Assistant Purdue University School of Materials Engineering 765/494-4100

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Seminar Reminder - Koppes, Mon. 7/16/12, 9:30 AM, ARMS Rm 3115
by Stacey, Lisa A 13 Jul '12

13 Jul '12

MATERIALS SCIENCE AND ENGINEERING SEMINAR Effect of Cyrystallographic Orientation on Hillock Formation in Thermally Cycled Large Grain Tin Films by John Koppes Ph.D. Final Exam Advisor Professor C. Handwerker ABSTRACT Tin whiskers and hillocks grow spontaneously from the surfaces of polycrystalline Sn films at room temperature. Whiskers can grow long enough to cause short circuits in electronic devices. We hypothesized that the anisotropies of the crystal structure lead to locally high strain energies that are relieved by the growth of whiskers and hillocks. This research studies hillock formations on large grain Sn-alloy films relative to the crystallographic orientations of the adjacent grains. Large grain films were produced by solidifying 96.5wt% Sn - 3wt% Ag - 0.5wt% Cu solder alloy on a Cu substrate. These surface defects (hillocks) grew predominately at grain boundaries during thermal cycling. The formation of the surface defects between two grains created a pseudo-bi-crystal sample geometry, making it ideal for studying surface defects relative to the local crystallographic orientations and the grains' corresponding anisotropic properties. The crystallographic orientations of the grains were studied with Electron Backscatter Diffraction (EBSD) and Laue micro-diffraction at the Lawrence Berkeley National Laboratory Advanced Light Source. Local orientation studies of the surface defects and the surrounding grains indicated that the surface defects nucleated and grew with low dislocation densities. In addition, the linear surface defect densities along the grain boundaries were measured and observed to change as a function of orientation. The change in linear defect density with respect to orientation was due, in part, to the anisotropy of the coefficient of thermal expansion of β-Sn. In addition, it was important to account for elastic anisotropies. The elastic stresses, strains, and strain energy densities of the microstructures were determined with Object Oriented Finite element analysis. The simulations indicated that during thermal cycling the local stresses exceeded the yield strength. As a result, the highest linear defect densities did not occur at orientations with the highest local strain energies. Instead, the highest local strain energies were accommodated by other plastic deformation mechanisms, indicating that surface defect formation is not only affected by local strain energy densities but also the local strain rates. Therefore, the propensity of specific grain orientations to form surface defects will change as a function of the thermal cycling process. Date: Monday, July 16, 2012 Time: 9:30 AM Place: ARMS 3115 Lisa Stacey Secretary/Development Assistant Purdue University School of Materials Engineering 765/494-4100

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Seminar Notice - El-Atwani, Wed. 7/18/12, 8:30 AM, ARMS Rm 3115
by Stacey, Lisa A 12 Jul '12

12 Jul '12

MATERIALS SCIENCE AND ENGINEERING SEMINAR Ion Beam Irradiation on Hard Material Surfaces: Nanopatterning of Gallium Antimonide and Silicon Substrates and Irradiation Damage of Ultrafine and Multimodal Tungsten by Osman Jihad El-Atwani Ph.D. Final Exam Advisor Professor J. P. Allain ABSTRACT Several important applications of ion beam irradiation emerged in the last two decades. While being utilized in investigating the irradiation damage to nuclear materials as plasma facing components (PFC), ion beam irradiation is also used in nanopatterning of single and multicomponent semiconductors. In this work, fundamental studies regarding the above two topics are presented. In the first study, formation of multimodal and ultrafine grain tungsten by spark plasma sintering is discussed and the irradiation damage of these materials at low energy irradiation (up to 200 eV) and high temperatures (up to 950 oC) is illustrated. Surface morphology changes and their correlation to grain size, grain boundary grooving, and irradiation enhanced recrystallization are discussed based on ex-situ irradiation/morphology observations. In the second study, several crucial aspects regarding nanostructuring of gallium antimonide (GaSb) surfaces via normal incident argon irradiation are discussed. In-situ surface characterization with x-ray photoelectron spectroscopy (XPS) and ion scattering spectroscopy (ISS) during the irradiation of GaSb surfaces at low energy (up to energies near sputtering threshold), was used to determine the surface concentration at different irradiation doses. Comparison with ex-situ characterized samples, where Ga concentrations were proved to be high due preferential reaction with oxygen, elucidated the significance of in-situ irradiation conditions. Moreover, several other aspects regarding the structures evolution and formation mechanism such as preferential sputtering of Sb, morphology evolution behavior, the effect of native oxide, and nanopatterning at energies near sputtering threshold are discussed based on in-situ XPS, ISS, and grazing incidence small angle x-ray scattering (GISAXS) results. In the third study, structuring of silicon substrates via normal incident ion irradiation is discussed. While ion beam irradiation of silicon substrates with no impurity seeding can lead to surface smoothing behavior, irradiation of metal-coated silicon substrates was shown to lead to nanodots formation on silicon substrates that remain even after the removal of the metal film material. Real time GISAXS and x-ray fluorescence (XRF) as well as in-situ XPS and ISS studies were performed during the irradiation of different metal-coated silicon substrates. The results, combined with ex-situ scanning electron microscopy (SEM), elucidated the importance of silicides in the structure formation mechanism. Date: Wednesday, July 18, 2012 Time: 8:30 AM Place: ARMS 3115 Lisa Stacey Secretary/Development Assistant Purdue University School of Materials Engineering 765/494-4100

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Seminar Notice - Efe, Tue. 7/17/12, 9:30 AM, ARMS Rm 3115
by Stacey, Lisa A 11 Jul '12

11 Jul '12

MATERIALS SCIENCE AND ENGINEERING SEMINAR Extrusion Machining: A Thermo-Mechanical Process for Producing Strips of Alloys Having Limited Workability by Mert Efe Ph.D. Final Exam Advisor Professor K. Trumble ABSTRACT Deformation processing of alloys with limited workability is one of the principal technological challenges for the metal manufacturing industry. Traditional processes (rolling, extrusion) are usually performed at high temperatures (0.5 Tm) for increased plasticity and crack-free products, which makes them costly, energy intensive and raw material inefficient. In this thesis, extrusion machining (EM) a shear-type deformation process is applied to alloys with limited workability for sheet and foil production from bulk forms in a single step. EM is shown to have a narrowly confined and controllable deformation zone, with attributes ranging from conventional deformation processing to severe plastic deformation. Controllable deformation parameters include strain, strain rate, hydrostatic pressure, temperature and deformation path. These attributes are highlighted in deformation processing of two classes of limited workability alloys: 1) Alloys with intrinsic susceptibility to cracking (and segmentation) due to flow localization and shear band formation and 2) Alloys with extrinsic susceptibility to cracking due to macroscopic defects such as porosity, casting defects and weak second phases. Magnesium AZ31 alloy is selected as a model material for the first class and sintered copper and cast brass alloys are selected for the second class. Noteworthy features of the process are suppression of segmentation by combinations of high hydrostatic pressures (p/2k = 1.7) and deformation temperatures (T > 0.5 Tm) realized by in-situ plastic heating, realization of a range of strains [cid:image006.png@01CD5F7F.0480AF00] and deformation rates [cid:image007.png@01CD5F7F.0480AF00] , engineering of microstructures ranging from conventional to ultrafine grained, and creation of sheet from the bulk in a single step of deformation without pre-heating. The thermo-mechanical conditions and microstructural processes that are responsible for continuous and sound sheet products are established. Guidelines for scalability and cost-effectiveness of EM for commercial production are analyzed and discussed. Date: Tuesday, July 17, 2012 Time: 9:30 AM Place: ARMS 3115 Lisa Stacey Secretary/Development Assistant Purdue University School of Materials Engineering 765/494-4100

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FW: Auto-discard notification
by Stacey, Lisa A 10 Jul '12

10 Jul '12

Lisa Stacey Secretary/Development Assistant Purdue University School of Materials Engineering 765/494-4100 -----Original Message----- From: mailman-bounces(a)ecn.purdue.edu [mailto:mailman-bounces@ecn.purdue.edu] On Behalf Of msepostdoc-list-bounces(a)ecn.purdue.edu Sent: Tuesday, July 10, 2012 2:20 PM To: msepostdoc-list-owner(a)ecn.purdue.edu Subject: Auto-discard notification The attached message has been automatically discarded.

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