Bulletin of the American Physical Society
APS March Meeting 2011
Volume 56, Number 1
Monday–Friday, March 21–25, 2011; Dallas, Texas
Session L28: Focus Session: Computational Materials Design - Property Optimization |
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Sponsoring Units: DCOMP DMP Chair: Stefano Curtarolo, Duke University Room: C156 |
Tuesday, March 22, 2011 2:30PM - 3:06PM |
L28.00001: Computational Design of Microstructures Invited Speaker: Many important engineering materials are designed by controlling their phase transformations and microstructure evolution. Examples include the improvement of mechanical properties through solid state precipitation reactions in alloys such as Ni-based superalloys and age-hardened Al-alloys, the useful dielectric properties and electro-mechanical coupling effects by manipulating the phase transitions in ferroelectric crystals, the memory effect of shape-memory alloys by utilizing martensitic transformations.~ In this presentation, recent effort on integrating the phase-field approach with other computational methods such as first-principles calculations and CALPHAD will be discussed. A number of examples of coupling phase-field simulations and experimental measurements will be presented. It will be demonstrated that one can use the phase-field method to not only help interpreting experimental observations but also provide guidance to achieve desirable transition temperatures and specific domain/microstructure structures. The possibility to directly obtain the effective responses of a microstructure under an applied field from phase-field simulations, and thus the evolution of effective mechanical and transport properties will also be discussed. [Preview Abstract] |
Tuesday, March 22, 2011 3:06PM - 3:18PM |
L28.00002: Phase-field model and its numerical solution for coring and microstructure evolution studies in alloys Patrice E. A. Turchi, Jean-Luc Fattebert, Milo R. Dorr, Michael E. Wickett, James F. Belak We describe an algorithm for the numerical solution of a phase-field model (PFM) of microstructure evolution in alloys using physical parameters from thermodynamic (CALPHAD) and kinetic databases. The coupled system of PFM equations includes a local order parameter, a quaternion representation of local crystal orientation and a species composition parameter. Time evolution of microstructures and alloy composition is obtained using an implicit time integration of the system. Physical parameters in databases can be obtained either through experiment or first-principles calculations. Application to coring studies and microstructure evolution of Au-Ni will be presented. [Preview Abstract] |
Tuesday, March 22, 2011 3:18PM - 3:30PM |
L28.00003: High Temperature Thermal Conductivity from First Principles Christian Carbogno, Ramamurthy Ramprasad, Matthias Scheffler In spite of significant research efforts, little is yet known about the atomistic details and mechanisms that underlie peculiarly low (or high) thermal conductivities, especially at elevated pressures and temperatures. Under such extreme conditions, systematic experimental measurements are hard to perform; conventional theoretical approaches typically fail to capture significant physical aspects of the problem, since these methods are either inherently limited to (a) low temperatures and/or (b) to perfect crystals. A recently developed {\it ab initio} simulation strategy~[1] allows to overcome the latter limitation, but the assessment of the high temperature regime remains an unsolved challenge. Within this work, we present efficient strategies to overcome this serious restriction and show their applicability for zirconia based ceramics - a material typically used in high temperature applications, for instance in thermal barrier coatings~[2].\\[0pt] [1] T. M. Gibbons, and S. K. Estreicher, {\it Phys. Rev. Lett.} {\bf 102}, 255502 (2009).\\[0pt] [2] D. R. Clarke, and C. G. Levi, {\it Annu. Rev. Mat. Res.} {\bf 33}, 383 (2003). [Preview Abstract] |
Tuesday, March 22, 2011 3:30PM - 3:42PM |
L28.00004: First Principles Studies of the Thermoelectric Figure of Merit of Zintl Compounds Ca$_{14}$AlSb$_{11-x}$As$_{x}$ Trinh Vo, Paul von Allmen, Jean-Pierre Fleurial We present predictions for the thermoelectric Figure of merit (ZT) of zintl compounds Ca$_{14}$AlSb$_{1-x}$As$_{x}$ obtained from Density Functional Theory calculations. The Seebeck coefficient, S, was obtained using the Boltzmann transport equation in the relaxation time approximation and first principles electronic structure calculations. We found that the Seebeck coefficient changes dramatically when one or more Sb atoms in the zintl compound Ca$_{14}$AlSb$_{11}$ are replaced with one or more As atoms, and that the difference in S between the original Ca$_{14}$AlSb$_{11}$ and the substituted one, Ca$_{14}$AlSb$_{11-x}$As$_{x}$, depends strongly on the positions of substituting As atoms. [Preview Abstract] |
Tuesday, March 22, 2011 3:42PM - 3:54PM |
L28.00005: First-principles model of absolute band shifts induced by (001) biaxial strain in group IIIA-VA semiconductors Eugene Kadantsev, Pawel Hawrylak A new model for the evolution of conduction and valence bands of IIIA-VA (InAs, GaAs, InP) semiconductors under (001) biaxial strain is developed. The model is based on ab initio calculations which take into account finite strain dependent relaxation of the reference levels. It is shown that in type I heterostructures subjected to (001) compressive biaxial strain, the confinement of holes can be reduced as compared to some existing models of biaxial strain. [Preview Abstract] |
Tuesday, March 22, 2011 3:54PM - 4:06PM |
L28.00006: Possible calcium centers for hydrogen storage applications: An accurate many-body study by AFQMC calculations with large basis sets Wirawan Purwanto, Henry Krakauer, Shiwei Zhang, Yudistira Virgus Weak H$_2$ physisorption energies present a significant challenge to first-principle theoretical modeling and prediction of materials for H storage. There has been controversy regarding the accuracy of DFT on systems involving Ca cations. We use the auxiliary-field quantum Monte Carlo (AFQMC) method\footnote{ S.~Zhang and H.~Krakauer, Phys.~Rev.~Lett.~\textbf{90}, 136401 (2003); W.~A.~Al-Saidi, S.~Zhang and H.~Krakauer, J.~Chem.~Phys.~\textbf{124}, 224101 (2006). } to accurately predict the binding energy of Ca$^+$\,-\,4{H}$_2$. AFQMC scales as $N_{\textrm{\small{basis}}}^3$ and has demonstrated accuracy similar to or better than the gold-standard coupled cluster CCSD(T) method. We apply a modified Cholesky decomposition to achieve efficient Hubbard-Stratonovich transformation in AFQMC at large basis sizes. We employ the largest correlation consistent basis sets available, up to Ca/cc-pCV5Z, to extrapolate to the complete basis limit. The calculated potential energy curve exhibits binding with a double-well structure. [Preview Abstract] |
Tuesday, March 22, 2011 4:06PM - 4:18PM |
L28.00007: Theoretical modification of $WO_3$ for water splitting Predrag Lazic, Maria K. Chan, Rickard Armiento, Yabi Wu, Gerbrand Ceder Using the sun's energy to produce hydrogen from water through photocatalytic process has been a dream since its first demonstration by Fujishima and Honda 40 years ago. Since then significant effort has been made to find a suitable material for this purpose but so far efficiency of the available materials is too low to be commercially interesting. However there are some promising candidates that have some very desirable properties for solar water splitting and their other properties are believed to be improvable by some changes in the material. One of such candidates is $WO_3$ which shows a very good light absorption and very high stability in aqueous environment. Unfortunately it also has a position of conduction band minimum slightly too low to support $H^{+}/H_2$ reaction of hydrogen evolution and also has a relatively large gap which prevents it from using a large part of solar spectrum and thus yielding a low efficiency for water splitting. We have tried to remedy those two problems by substitutions and codoping in the pure $WO_3$ material within the density functional theory. For some of the modifications we see improved material properties. [Preview Abstract] |
Tuesday, March 22, 2011 4:18PM - 4:30PM |
L28.00008: Ab initio investigations of complex oxides Altynbek Murat, Julia E. Medvedeva We employ {\it ab-initio} density functional approach to investigate the structural, optical and electronic properties of twelve complex oxides with layered structure RAMO$_4$, R=In or Sc, A=Al, Ga, M=Ca, Cd, Mg, and/or Zn. We find that presence of the light metal (e.g., Al, Ca, Mg and Sc) oxides significantly affects the optical band gap which varies from 0.64 eV (InGaCdO$_4$) to 4.35 eV (ScAlMgO$_4$). At the same time, the electron effective mass remains nearly isotropic in all oxides, and both structurally and chemically distinct layers are expected to participate in charge transport once the materials are degenerately doped. Further, for a comparative systematic investigation of carrier generation mechanisms in complex oxides, we calculated the electronic properties of fluorine doped (F$_O$) and oxygen-reduced RAMO$_4$ materials as well as their single-cation constituents in various phases. We determine most preferable spatial distribution of the F impurity and the oxygen defect in the layered structure of each material and find that the dopant/vacancy site locations correlate with the formation energy of the single-cation oxides. The results allow us to draw conclusions on the role played by each constituent oxide and to predict how the properties of multicomponent materials can be controlled via chemical composition, crystal structure and carrier generation. [Preview Abstract] |
Tuesday, March 22, 2011 4:30PM - 4:42PM |
L28.00009: Tuning of Metal-Metal Bonding by Counterion Size in Hypothetical AETiO2 Compounds Xiaodong Wen, Roald Hoffmann The structures and electronic properties of a number of real and hypothetical ABX2 compounds sharing (or evolving from) a single P4/mmm structural type are examined utilizing first principle calculations. These include the known CaCuO2 and SrFeO2 phases. A number of variations of this P4/mmm ABX2 framework, some obvious, some exotic, all with a chemical motivation, were investigated: A=alkali metal, alkaline earth metal or La, B= Ti, Fe, Cu or Pt, and X=C, O, S, C2, H2 or F. Careful attention was given to the d-orbital splitting patterns and magnetic states (ferromagnetic or antiferromagnetic) of these compounds, as well as their stability gauged by phonon dispersions and energetics. The most interesting as yet unmade compounds that emerged was an AETiO2 (AE = alkaline earth metal, Be, Mg, Ca, Sr and Ba) series, with Ti-Ti bonding, part $\sigma $, part $\pi $, tuned by the AE2+ cation size. The Ti-Ti bonding in 3D AETiO2 structures has a unique electronic feature of 1D metal chain. These AETiO2 (M=Ca, Sr and Ba) structures are calculated to be thermodynamically and dynamically stable. Experimentally, the high temperature method fails so far. Perhaps a low temperature method offer a better pathway to synthesize the AETiO2 (M=Ca, Sr and Ba) structures. [Preview Abstract] |
Tuesday, March 22, 2011 4:42PM - 4:54PM |
L28.00010: Rational Band Structure Engineering of TiO$_{2}$ for Photoelectrochemical Water Splitting Su-Huai Wei, Wan-Jian Yin, Yanfa Yan The search for new semiconducting materials or the engineering of existing semiconductors for commercially viable photoelectrochemical (PEC) water splitting has been extremely challenging. Meeting that challenge requires the discovery of a semiconductor with several tightly coupled material property criteria such as appropriate band gap (1.6 -- 2.2 eV), efficient visible light absorption, high carrier mobility, and correct band edge positions that straddle the water redox potentials. However, previous searches/modifications of semiconducting materials for PEC water splitting application have often focused on a particular individual criterion such as band gap, neglecting the possible detrimental consequence to other important criteria. In this talk, general strategies for the rational design of semiconductors such as TiO$_{2}$ to simultaneously meet all of the requirements for a high efficiency solar-driven PEC water splitting device are discussed. Density-functional theory calculations reveal that with appropriate donor-acceptor co-incorporation, heavily doped anatase TiO$_{2}$ hold great potential to satisfy all of the criteria for a viable PEC device. Other approaches to modify the band structure of TiO$_{2}$, such as the application of strain, will also be discussed. [Preview Abstract] |
Tuesday, March 22, 2011 4:54PM - 5:06PM |
L28.00011: Structural and Electronic Properties of LaTiO$_2$N with O/N Disorder Wei Kang, Mark S. Hybertsen LaTiO$_2$N is an attractive candidate photo-catalyst for water-splitting, showing strong absorption in the visible range with an optical gap about 2.1 eV and catalytic activity for hydrogen and oxygen evolution in the presence of auxiliary co-catalysts. It is also a good prototype suitable for theoretical study. It has a small unit cell while exhibiting several key characteristics found in the more complex oxides and oxynitrides synthesized in the search for improved photo-catalysts. This includes the reduced band gap and the disorder in one of the components, the O/N anion sublattice. We study the structural properties using a first-principles cluster expansion method. Our results reveal that at the temperatures characteristic of synthesis and annealing conditions, the occupation of O/N in LaTiO$_2$N is intrinsically disordered. However the structure retains residual long-range order, in agreement with anion site occupancies measured in powder neutron diffraction. Short-range order in the O/N occupation is also observed. We use many-body perturbation theory to study the electronic and optical properties for some low-energy structures. The fundamental gap is found about 0.5 eV lower than the apparent absorption edge observed in experiments. [Preview Abstract] |
Tuesday, March 22, 2011 5:06PM - 5:18PM |
L28.00012: The Se effect on the oxygen reduction reaction on the Se/Ru electro-catalysts. Insight from first principles. Sergey Stolbov Rational search for new efficient low-cost electrocatalysts for oxygen reduction reaction (ORR) on the hydrogen fuel cell cathodes focuses on varying the material composition to modify the local densities of electronic states (LDOS) of the surface atoms, in order to tune the surface-adsorbate electronic state hybridization and hence binding energies of the ORR intermediates. My calculation results for the Se/Ru electrocatalysts suggest an alternative way of tuning the binding energies. The Se atoms deposited on the Ru surface are found not to change Ru LDOS noticeably, however, Se atoms are negatively charged due to ionic Se-Ru bonding. As a result, they repeal electrostatically the adsorbed negatively charged O and OH intermediates, and this way reduce their binding energies. Since for the Ru case, reduction of the O and OH binding energies makes ORR energetically favorable, Se deposition dramatically improve the ORR rate on Ru. The ORR rate can thus be enhanced by changing coverage of the deposited halchogen atoms or by tuning the charge transfer to those by modifying the substrate composition. [Preview Abstract] |
Tuesday, March 22, 2011 5:18PM - 5:30PM |
L28.00013: First principles studies of the oxygen reduction reaction on Se-Ru nanostructures Sebastian Zuluaga, Sergey Stolbov Experiments show an enhanced rate of the oxygen reduction reaction (ORR) on Se-Ru nanostructures (NS) in hydrogen fuel cell cathodes. We use first principles methods to study Ru and Se-Ru NS of approximate 1.2 nm size and shine some light on how the Se affects the O and OH adsorption, which is the bottle neck of the power delivered by the fuel cell. Experiments shows that the Se-Ru NS have a Ru core but is not clear how the Se is distributed on the surface. Our calculation shows that the Se atom adsorbs on the Ru surface with a binding energies in the range 5.7 to 7.1 eV with electronic charge transfer from the Ru atoms. Due to repulsion between negatively charged Se atoms, they tend to spread uniformly over the the Ru NS rather than form islands on its surface. We have also found that, in contrast to the flat Ru surface, the Se bond to the low coordinated Ru atoms have significant covalent component. Our calculation shows how the presence of Se atoms affects the adsorption of the ORR intermediates on the NS. In particular, we show that the electrostatic repulsion between charged Se and O or OH reduces the binding energy of the latters. [Preview Abstract] |
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