Bulletin of the American Physical Society
APS March Meeting 2011
Volume 56, Number 1
Monday–Friday, March 21–25, 2011; Dallas, Texas
Session X32: Focus Session: Frontiers in Computational Thermodynamics of Materials II |
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Sponsoring Units: FIAP DCOMP Chair: Stefano Curtarolo, Duke University Room: C144 |
Thursday, March 24, 2011 2:30PM - 3:06PM |
X32.00001: Low temperature phase transition predicted in the compound B13C2/B4C Invited Speaker: The experimental phase diagram of boron-carbon exhibits the compound boron-carbide over a broad composition range that extends to low temperatures, in seeming contradiction to the third law of thermodynamics. First principles total energy calculations suggest the presence of two energy-minimizing structures in the boron-carbon phase diagram, B13C2 and B4C. Both distribute boron and carbon atoms on the same 15-atom rhombohedral unit cell (hR15), consisting of 12-atom icosahedra at cell vertices plus three-atom chains at cell centers. However, only B13C2 respects the rhombohedral symmetry, while B4C breaks the symmetry by replacing one of the icosahedral boron atoms with carbon. Because B4C is incompatible with the experimentally observed rhombohedral symmetry, it must lose thermodynamic stability at elevated temperatures. We report a study of the configurational ensemble obtained by substitution of boron or carbon on different sites using a semi-grand canonical ensemble. Varying chemical potential at low temperature, we find sharp transitions from beta-rhombohedral boron to B13C2 then to B4C and finally to graphitic carbon. Only the rhombohedral-symmetry phase B13C2 survives at high temperature while the symmetry-broken phase B4C loses stability around room temperature. [Preview Abstract] |
Thursday, March 24, 2011 3:06PM - 3:18PM |
X32.00002: Thermodynamic modeling of Pt-Al and Pd-Al Derek Carr Pure platinum and pure palladium are too soft for typical jewelry applications. Adding small amounts of other metals can significantly increase their performance. However, international hallmarking standards require the alloys to be 95{\%} pure by weight. How does one achieve significant improvements in performance adding only small amounts (5 wt-{\%}) of other metals? Significant improvements are possible even with small additions if precipitate hardening can be induced. Using a combination of first-principles, cluster expansion, and Monte Carlo modeling, we have identified new Pt-rich/Pd-rich phases in Pt-Al and Pd-Al that should be useful in precipitate hardening. Thermodynamical modeling indicates that the phases are experimentally feasible (not kinetically inhibited). [Preview Abstract] |
Thursday, March 24, 2011 3:18PM - 3:30PM |
X32.00003: Equation of State and Viscosity of Tantalum and Iron from First Principles Ljubomir Miljacic, Steven Demers, Axel van de Walle To understand and model at continuum level the high-energy-density dynamic response in transition metals like Tantalum and Iron, as it arises in hypervelocity impact experiments, an accurate prediction of the underlying thermodynamic and kinetic properties for a range of temperatures and pressures is of critical importance. The relevant time scale of atomic motion in a dense gas, liquid, and solid is accessible with \textit{ab-initio} Molecular Dynamics (MD) simulations. We calculate EoS for Ta and Fe via Thermodynamical Integration in 2D (V,T) phase space throughout different single and two-component phases. To reduce the \textit{ab-initio} demand in selected regions of the space, we fit available gas-liquid data to the Peng-Robinson model [1] and treat the solid phase within the Boxed-quasi-harmonic approximation [2]. In the fluid part of the 2D phase space, we calculate shear viscosity via Green-Kubo relations, as time integration of the stress autocorrelation function. \\[4pt] [1] Ind. Eng. Chem., Fundam \textbf{15}, 59 (1976) \\[0pt] [2] A. van de Walle and G. Ceder, \textit{Rev. Mod. Phys.} \textbf{74} 11 (2002) [Preview Abstract] |
Thursday, March 24, 2011 3:30PM - 3:42PM |
X32.00004: Binary Magnesium Alloys: Searching for Novel Compounds by Computational Thermodynamics Richard Taylor, Stefano Curtarolo, Gus Hart Magnesium alloys are among the lightest structural materials and are of considerable technical interest. We use the high-throughput framework AFLOW to make T = 0 K ground state predictions by scanning a large set of known candidate structures for thermodynamic minima. The study presented here encompasses 34 Mg-X systems of interest (X=Al, Au, Ca, Cd, Cu, Fe, Ge, Hg, Ir, K, La, Pb, Pd, Pt, Mo, Na, Nb, Os, Rb, Re, Rh, Ru, Sc, Si, Sn, Sr, Ta, Tc, Ti, V, W, Y, Zn, Zr). Avenues for further investigation revealed by this study include stable phases found in addition to experimental phases and compound forming systems thought to be either immiscible or non-compound forming. The existence of potentially novel ordered phases presents new opportunities for materials design. [Preview Abstract] |
Thursday, March 24, 2011 3:42PM - 3:54PM |
X32.00005: High-throughput combinatorial search of novel topological insulators Kesong Yang, Wahyu Setyawan, Shidong Wang, Jeffrey Mulllen, Marco Buongiorno-Nardelli, Stefano Curtarolo In recent years, topological insulators (TIs) have attracted lots of attentions not only because of their interesting electronic characteristics induced by spin-orbit coupling but also their potential applications. So far, experimentally observed topological insulators mainly include HgTe/CdTe quantum well structure, semiconducting alloy Bi$_{1-x}$Sb$_x$, and so-called second-generation TI materials, i.e., the family of Bi$_2$Se$_3$, Bi$_2$Te$_3$, and Sb$_2$Te$_3$. Later theoretical simulations predict more TIs such as TlBiQ$_2$ and TlSbQ$_2$ (Q=Te, Se, and S), LaBiTe$_3$ as well as half-Heusler alloys, LuPtSb and ScPtBi. Numerous attempts are being made to look for more TIs. In this presentation, we will introduce our high-throughput combinatorial approach to find novel TI materials based on the AFLOW framework and distributed libraries. [Preview Abstract] |
Thursday, March 24, 2011 3:54PM - 4:06PM |
X32.00006: Classification of the ICSD by crystal Prototype Junkai Xue, Wahyu Setyawan, Stefano Curtarolo An efficient way to determine the prototypes of the ICSD database has been implemented within our high-throughput formalism. This presentation illustrates how we use this tool to search for structure/properties correlations in an automatic fashion. [Preview Abstract] |
Thursday, March 24, 2011 4:06PM - 4:18PM |
X32.00007: Novel Occurences of L1$_1$ and L1$_3$ found using the synnergy between High Throughput and Cluster Expansion Lance Nelson, Gus Hart, Stefano Curtarolo Despite their geometric simplicity, L1$_1$(CuPt) and L1$_3$ (CdPt$_3$) fail to appear as groundstates in experimental systems. ( L1$_1$ appears in CuPt only) Are these crystal structures actually energetically unfavorable, or have they simply been overlooked in experimental studies? Here we investigate, using computational methods, the energetic stability of these phases in all binary inter-metallic systems. We combine the results of two techniques,namely High Throughput (HT) and Cluster Expansion (CE), to maximize efficiency and ensure thoroughness. HT results show L1$_1$(L1$_3$) to be stable in the following systems: AgPd, AgPt, CuPt, PdPt(CdPt,CuPt,PdPt,LiPd,LiPt). Cluster expansions constructed for these systems verify the HT findings in all cases, with the exception of the HT groundstate PdPt-L1$_1$.(D$_4$ is found to be energetically more favorable) Monte Carlo simulations, which are used to identify order-disorder transition temperatures, were performed for all occurences of these two phases. While the transition temperatures for some systems are found to be extremely low, others appear to be physically realizable. [Preview Abstract] |
Thursday, March 24, 2011 4:18PM - 4:30PM |
X32.00008: Ab Initio Insights on the Shapes of Nanocrystals Roman Chepulskyy, Stefano Curtarolo Catalytic, chemical, optical and electronic properties of nanocrystals are strongly influenced by their faceting. A variational approach based on quantum mechanical energies is introduced to evaluate stable and metastable shapes of nanocrystals. The method leads to a nanoscale equation of state, which is solved self-consistently. Using platinum as example, it is found that the surface energy dependence on the lattice parameter is the key factor controlling the equilibrium stability of the crystal shapes. The energies of different surfaces versus lattice parameter are calculated from first principles in high-throughput fashion. Considering several crystal shapes and using Wulff's construction, the transitions between stable and metastable shapes are predicted below 3 nm in diameter. Our variational approach explains experimental results and establishes a direction to search for better catalysts. [Preview Abstract] |
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