Bulletin of the American Physical Society
Annual Meeting of the Four Corners Section of the APS
Volume 55, Number 9
Friday–Saturday, October 15–16, 2010; Ogden, Utah
Session C3: Condensed Matter, Structure |
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Chair: Colin Inglefield, Weber State University Room: 404B |
Friday, October 15, 2010 2:00PM - 2:24PM |
C3.00001: Neutron Diffraction Studies on Gold Crystals from Placer Deposits Invited Speaker: The single-crystal nature of large (centimeter scale) gold crystals from placer deposits in Venezuela, Russia and elsewhere had often been questioned. Testing whether those gold crystals are indeed single crystalline has been a challenge because sectioning crystals to expose their interiors is both undesirable and problematic in that cutting or grinding creates significant deformation. Because of the relatively small penetration depth, X-rays will test mainly the surface structure. While in most cases the X-ray diffraction patterns on such crystal are inconsistent with single-crystal patterns, it has been hypothesized that surfaces may be damaged due to cold working as a result of mechanical distortion associated with stream transport. Unlike X-rays, neutrons can penetrate deep into the bulk of a material. Our neutron-diffraction data on selected specimens confirmed the single-crystalline nature of some of the crystals from Venezuela, while the Russian crystals were found to be cast imitations. [Preview Abstract] |
Friday, October 15, 2010 2:24PM - 2:36PM |
C3.00002: New structures in Pd-rich ordered alloys Jacqueline Corbitt, Erin Gilmartin, Gus Hart An intriguing intermetallic structure with 8:1 stoichiometry was discovered in the 1950s in the Pt-Ti system. Since then a handful of other Pt/Pd/Ni binary systems have been observed to exhibit this curious structure (Pt$_8$Zr, Pd$_8$Mo, Ni$_8$Nb, etc). This ordered structure can significantly increase the hardness of an alloy. For jewelry applications involving Pt and Pd, international hallmarking standards require that the alloys be at least 95\% pure by weight. However, Pt- and Pd-rich alloys are often soft when purity is high if the minority atoms are disordered. Because the 8:1 structure maintains a high weight percentage of Pt/Pd, it can satisfy purity standards while increasing performance. Recent calculations and experiments suggest that the 8:1 structure may form in about 20 previously unsuspected Pt/Pd binary systems. Using first-principles calculations and cluster expansion modeling, we have performed a ground state search to find the stable structures in Pd-Nb and Pd-Cu. In collaboration with Candace Lang's group at University of Capetown South Africa, we are working to experimentally validate the predicted ground states. [Preview Abstract] |
Friday, October 15, 2010 2:36PM - 2:48PM |
C3.00003: The experimental search for new predicted binary-alloy structures K.C. Erb, Lauren Richey, Candace Lang, Branton Campbell, Gus Hart 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?,'' \textit{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 metallic alloys. Many of these phases have been observed experimentally in the past, though others have not. In pursuit of several of the missing structures, we have characterized potential orderings in PtCd, PtPd and PtMo alloys using synchrotron x-ray powder diffraction and symmetry-analysis tools.\footnote{B. J. Campbell, H. T. Stokes, D. E. Tanner, and D. M. Hatch, ``ISODISPLACE: a web-based tool for exploring structural distortions,'' \textit{J. Appl. Cryst}. 39, 607-614 (2006)} [Preview Abstract] |
Friday, October 15, 2010 2:48PM - 3:00PM |
C3.00004: Pair distribution function analysis on pyrolyzed porphyrin Joe Peterson Transition metal-porphyrins are well known to be effective catalysts. One of particular interest in research surrounding fuel cells is CoTMPP because it facilitates 4e- oxidation reduction reactions (ORR). However, under conditions found in proton exchange membrane fuel cells (PEMFC) the catalytic properties begin to breakdown and ORR shifts toward 2e-. This negative effect can be largely overcome by pyrolyzing the CoTMPP. Though the pyrolysis improves the catalytic properties, it introduces uncertainty in site activity. Extensive efforts with TEM and XPS have been taken to better understand the structure, but still ambiguity of active sites persists. Here we present addition studies with pair distribution function analysis to aid in the growing understanding of the structure of the pyrolyzed porphyrin. [Preview Abstract] |
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