Bulletin of the American Physical Society
2011 Annual Meeting of the Four Corners Section of the APS
Volume 56, Number 11
Friday–Saturday, October 21–22, 2011; Tuscon, Arizona
Session M3: Materials |
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Chair: John R. Dennison, Utah State University Room: UA Student Union Agave |
Saturday, October 22, 2011 11:10AM - 11:22AM |
M3.00001: Electron Energy Dependent Charging Effects of Multilayered Dielectric Materials Gregory Wilson, Amberly Evans, J.R. Dennison Measurements of the charge distribution in electron-bombarded, thin-film, multilayer dielectric samples showed that charging of multilayered materials evolves with time and is highly dependent on incident energy; this is driven by electron penetration depth, secondary electron emission and material resistivity. A thin film SiO$_{2}$ structure with a conductive middle layer was charged by bombardment using a 200 eV to 5 keV electron beam with regular 15 s pulses at 1 nA/cm$^{2}$ to 500 nA/cm$^{2}$. Measurements of the surface potential, displacement current and the beam energy allow the charge distribution to be inferred based on the net surface potential's dependence on electron range, beam current and secondary electron emission. Results are shown to be consistent with simple models, using previous results of electron range, yields and resistivity. Large negative net surface potentials led to electrostatic breakdown and large visible arcs which have been observed to lead to detrimental spacecraft charging effects. The project was funded by a grant from NASA Goddard Space Flight Center. [Preview Abstract] |
Saturday, October 22, 2011 11:22AM - 11:34AM |
M3.00002: The Experimental Search for New Predicted Binary-alloy Structures K.C. Erb, Lauren Richey, Branton Campbell, Gus Hart, Candace Lang Predicting new ordered phases in metallic alloys is a productive line of inquiry because configurational ordering in an alloy can dramatically alter their useful material properties. One is able to infer the existence of an ordered phase in an alloy using first-principles calculated formation enthalpies.\footnote{G. L. W. Hart, ``Where are Nature's missing structures?'' Nature Materials 6 941-945 2007} Using this approach, we have been able to identify stable (i.e. lowest energy) orderings in a variety of binary Pt-based alloys, many of which have never before been observed experimentally. After preparing alloys of the desired composition, we performed synchrotron x-ray powder diffraction experiments to prove or disprove the expected orderings. [Preview Abstract] |
Saturday, October 22, 2011 11:34AM - 11:46AM |
M3.00003: Modeling of Amorphous Calcium Carbonate Sourabh Sinha, Peter Rez Many species (e.g. sea urchin) form amorphous calcium carbonate (ACC) precursor phases that subsequently transform into crystalline CaCO$_{3}$. It is certainly possible that ACC might have up to 10 wt{\%} Mg and $\sim $3 wt{\%} of water. The structure of ACC and mechanisms by which it transforms to crystalline phase are still unknown. Our goal here is to determine an atomic structure model that is consistent with diffraction and IR measurements of ACC. For this purpose a calcite supercell with 24 formula units (120 atoms) was constructed. Various configurations with 6 Mg atoms substituting for Ca (6 wt{\%}) and 3-5 H$_{2}$O molecules (2.25- 3.75 wt{\%}) inserted in the spaces between Ca atoms, were relaxed using VASP. Most noticeable effects were the tilts of CO$_{3}$ groups and distortion of Ca sub-lattice, especially in the case of water. The distributions of nearest Ca-Ca distance and CO$_{3}$ tilts were extracted from those configurations. We also performed the same analysis starting with aragonite. Sampling from above distributions we built models for amorphous calcite/aragonite of size $\sim $1700 nm$^{3}$. We found that the induced distortions were not enough to generate a diffraction pattern typical of an amorphous material. Next we studied diffraction pattern of several nano-crystallites as recent studies suggest that amorphous calcite might be composed of nano- crystallites. We could then generate a diffraction pattern that appeared similar to that from ACC, for a nano-crystallite of size $\sim $2 nm$^{3}$. [Preview Abstract] |
Saturday, October 22, 2011 11:46AM - 11:58AM |
M3.00004: Defect-induced diffuse scattering in microporous aluminophosphate-5 Nichole M. Maughan, Sumner Norman, Daniel G. Robertson, Branton J. Campbell Microporous framework materials, the most common type being aluminosilicate zeolites, are crystalline compounds with interconnected networks of channels and cavities through which ions and molecules can flow. Aluminophosphate-5 (AlPO-5) is a zeolite analog in which alternating aluminum and phosphorus atoms occupy the tetrahedral sites of the AFI framework type. AlPO-5 is useful as a molecular sieve, especially when doped with chromium. Our single-crystal X-ray diffuse scattering (SCXDS) data from AlPO-5 reveals evidence of planar framework defects that have the potential to alter the material's useful properties. By generating candidate defect models, simulating their diffuse scattering patterns, and comparing them against experimental data, we aim to establish the correct model. [Preview Abstract] |
Saturday, October 22, 2011 11:58AM - 12:10PM |
M3.00005: Fuel from Water: Nano-structured oxide semiconductor composites for photocatalysis Doug Ball, Fred Walker, Joe Ngai Photo-catalytic hydrogen production has been observed using TiO2 as a catalyst to decompose water but has proved impractical in application. The inefficiency of TiO2 is a result of its large bandgap which only permits absorption in the UV range of light and not visible light. Complex oxide-semiconductors are potentially a more effective candidate for photocatalysis of water in that they provide both the electrons from a smaller band gap semiconductor and the catalyst from the oxide surface. We examine an oxide-semiconductor composite of BaTiO3 grown on Ge using the Molecular Beam Epitaxy (MBE) technique by measuring electrical transport to understand the physics of photo-generated carriers. Understanding these electrical transport characteristics will assist in optimizing the efficiency of the nanostructured composite for its eventual use in a reduction reaction of water. The photo-generated surface electrons are observed when the composite is placed in AgNO3 solution and irradiated with visible light. Ag+ reduces because of the available surface electrons forming Ag [solid] grains which are deposited on the surface and observed using a scanning electron microscope with energy dispersive X-ray spectroscopy. [Preview Abstract] |
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