68 research outputs found
Thermal expansion of Ir, Re, Ru, and Ru alloys
No full textThis dataset contains thermal expansion calculations of pure Ir, Re, Ru, and binary/ternary Ru alloys with special quasi-random structures. Publication: Ruoshi Sun, Mark Asta, Axel van de Walle. First-principles thermal compatibility between Ru-based Re-substitute alloys and Ir coatings. Computational Materials Science. Volume 170, 2019, ISSN 0927-0256. https://doi.org/10.1016/j.commatsci.2019.109199.The authors acknowledge support from the Office of Naval Research through program number N00014-16-1-3124. Computational resources were provided by the Center for Computation and Visualization at Brown University and the Extreme Science and Engineering Discovery Environment (XSEDE) Stampede2 at the Texas Advanced Computing Center through allocation TG-DMR050013N, which is supported by National Science Foundation Grant No. ACI-1548562
Thermal expansion of Ir, Re, Ru, and Ru alloys
No full textThis dataset contains thermal expansion calculations of pure Ir, Re, Ru, and binary/ternary Ru alloys with special quasi-random structures. Publication: Ruoshi Sun, Mark Asta, Axel van de Walle. First-principles thermal compatibility between Ru-based Re-substitute alloys and Ir coatings. Computational Materials Science. Volume 170, 2019, ISSN 0927-0256. https://doi.org/10.1016/j.commatsci.2019.109199.The authors acknowledge support from the Office of Naval Research through program number N00014-16-1-3124. Computational resources were provided by the Center for Computation and Visualization at Brown University and the Extreme Science and Engineering Discovery Environment (XSEDE) Stampede2 at the Texas Advanced Computing Center through allocation TG-DMR050013N, which is supported by National Science Foundation Grant No. ACI-1548562
Feasibility of band gap engineering of pyrite FeS₂
No full textWe use first-principles computations to investigate whether the band gap of pyrite FeS₂ can be increased by alloying in order to make it a more effective photovoltaic material. In addition to the isostructural compounds that have a larger band gap (ZnS₂, RuS₂, OsS₂), we evaluate non-rare-earth isovalent alloying candidates among all metals, transition metals, and semiconductor elements up to group IV and period 6 in the periodic table. From this screening procedure, we find that the group II elements (Be, Mg, Ca, Sr, Ba) and Cd have higher band gaps in the pyrite structure than FeS₂. Practical band gap enhancement is observed only in the Ru and Os alloyed systems, but their incorporation into pyrite may be severely limited by the large positive enthalpy of mixing. All other candidate (Fe,M)S₂ systems exhibit very large gap bowing effects such that the band gap at intermediate compositions is even lower than that of FeS₂. Positive correlations between immiscibility and differences in electronegativity and Shannon ionic radius are observed.United States. Dept. of Energy (Contract No. DE-FG02-05ER46253)Chesonis Family Foundation (Solar Revolution Project Fellowship
Stability maps to predict anomalous ductility in B2 materials
No full textWhile most B2 materials are brittle, a new class of B2 (rare-earth) intermetallic compounds is observed to have large ductility. We analytically derive a necessary condition for ductility (dislocation motion) involving ⟨111⟩ versus ⟨001⟩ slip and the relative stability of various planar defects that must form. We present a sufficient condition for antiphase boundary bistability on {1[bar over 1]0} and {11[bar over 2]} planes that allows multiple slip systems. From these energy-based criteria, we construct two stability maps for B2 ductility that use only dimensionless ratios of elastic constants and defect energies, calculated via density functional theory. These two conditions fully explain and predict enhanced ductility (or lack thereof) for B2 systems. In the 23 systems studied, the ductility of YAg, ScAg, ScAu, and ScPd, ductile-to-brittle crossover for other rare-earth B2 compounds, and brittleness of all classic B2 alloys and ionic compounds are correctly predicted.United States. Dept. of Energy. Division of Materials Sciences and Engineering (Grant DEFG02-03ER46026
First-principles electronic structure and relative stability of pyrite and marcasite: Implications for photovoltaic performance
No full textDespite the many advantages (e.g., suitable band gap, exceptional optical absorptivity, earth abundance) of pyrite as a photovoltaic material, its low open-circuit voltage (OCV) has remained the biggest challenge preventing its use in practical devices. Two of the most widely accepted reasons for the cause of the low OCV are (i) Fermi level pinning due to intrinsic surface states that appear as gap states, and (ii) the presence of the metastable polymorph, marcasite. In this paper, we investigate these claims, via density-functional theory, by examining the electronic structure, bulk, surface, and interfacial energies of pyrite and marcasite. Regardless of whether the Hubbard U correction is applied, the intrinsic {100} surface states are found to be of dz2 character, as expected from ligand field theory. However, they are not gap states but rather located at the conduction-band edge. Thus, ligand field splitting at the symmetry-broken surface cannot be the sole cause of the low OCV. We also investigate epitaxial growth of marcasite on pyrite. Based on the surface, interfacial, and strain energies of pyrite and marcasite, we find from our model that only one layer of epitaxial growth of marcasite is thermodynamically favorable. Within all methods used (LDA, GGA-PBE, GGA-PBE+U, GGA-AM05, GGA-AM05+U, HSE06, and delta-sol), the marcasite band gap is not less than the pyrite band gap, and is even larger than the experimental marcasite gap. Moreover, gap states are not observed at the pyrite-marcasite interface. We conclude that intrinsic surface states or the presence of marcasite are unlikely to undermine the photovoltaic performance of pyrite.United States. Dept. of Energy (contract DE-FG02-96ER45571)National Science Foundation (U.S.) (TeraGrid resources provided by Texas Advanced ComputingCenter (TACC) under grant TG-DMR970008S.
Intrinsic stoichiometry and oxygen-induced p-type conductivity of pyrite FeS2
No full textThe stoichiometry and ubiquitous observation of p-type conductivity of synthetic pyrite FeS[subscript 2] thin films are investigated via first-principles computations of native (vacancies, interstitials, antisites) and extrinsic (O[subscript S], O[subscript i]) point defects. Native defects have high formation energies and are predicted to occur in low concentrations within the Fe- and S-rich limits, showing that pyrite should be intrinsically stoichiometric. Under sufficiently oxidizing conditions, O[subscript S] becomes the most dominant defect type and induces p-type conductivity. At the experimental oxygen impurity concentration, the hole concentration is predicted to be O(10[superscript 19]) cm[superscript −3], in agreement with Hall measurements reported in the literature. Therefore, we attribute the unintentional p-type conductivity of pyrite to oxygen impurities and propose that improvements in device performance may be achieved under more reducing conditions.National Science Foundation (U.S.) (Grant No. TG-DMR970008S
Photovoltaic properties and size-pH phase stability of iron disulfide from density-functional theory
No full textThesis: Ph. D., Massachusetts Institute of Technology, Department of Materials Science and Engineering, 2013.This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.Cataloged from student-submitted PDF version of thesis.Includes bibliographical references (pages 105-114).Despite its exceptional optical absorptivity, suitable band gap, and earth abundance, the low open-circuit voltage of pyrite FeS₂ has remained the biggest challenge preventing its use in photovoltaic devices. Two widely-accepted causes are: (i) Fermi level pinning caused by intrinsic surface states that appear as gap states; (ii) presence of the polymorph marcasite. Based on density-functional theory (DFT) calculations, (i) the intrinsic (100) surface states are not gap states but located at the conduction band edge; (ii) epitaxial growth of marcasite on pyrite is thermodynamically favorable, but its band gap (from Kohn-Sham [delta]-sol method) is not less than pyrite. It is unlikely that the photovoltaic performance of pyrite is undermined by intrinsic surface states or marcasite. The stoichiometry and the ubiquitous observation of unintentional p-type conductivity of pyrite thin films are investigated via DFT defect computations. Native defects occur in low concentrations due to high formation energies, implying that pyrite is intrinsically stoichiometric. The p-type conductivity can be caused by OS defects under oxidizing conditions. Band gap engineering of pyrite is studied by alloying with non-rare-earth isovalent elements via DFT computations. We identify six MS₂ candidates that have larger band gaps than pyrite. Band gap enhancement of pyrite is observed only in the Ru and Os alloyed systems, but their incorporation into pyrite may be severely limited. All other candidate alloys exhibit large gap bowing effects due to size and/or electronegativity mismatch. The effects of particle size and pH on the relative phase stability of pyrite and marcasite polymorphs are explored. The size effect is incorporated through volume scaling of Wulff shapes. The pH effect is modeled by generalized, charged surface energies as a result of ion adsorption from the aqueous environment. Based on joint density-functional theory calculations, pyrite is unstable in highly acidic conditions due to a negative H+-adsorbed (110) surface energy, but stabilized for pH >/~ 2. Directions for future work are briefly discussed.by Ruoshi Sun.Ph. D
First-Principles Design Maps for Predicting Anomalous Ductility in B2 Materials
No full textRecently, a new class of B2 intermetallic compounds, e.g., YAg and YCu, were experimentally shown to have significant ductility comparable to face-centered cubic (fcc) Al, in contrast to other B2 materials like CuZn or NiAl. As of yet, there has been no explanation for the enhanced ductility in this class of materials. In order to provide understanding and a means to predict for such behavior, we derive, using mesoscale dislocation mechanics, criteria of h111i slip versus h001i slip, as well as the relative stability of antiphase boundaries (APB) and stacking faults (SF), together giving the necessary condition for ductility. Combined with the sufficient condition, which requires APB bistability on f1¹10g and f11¹2g planes, stability maps are constructed using dimensionless ratios of the calculated lattice constants, elastic constants, a 2 h111i f1¹10g and a 2 h111i f11¹2 g APB energies, and a2 h001i f1¹10g SF energies. To obtain required input to the stability maps, we have performed ¯rst-principles density-functional theory (DFT) calculations on three types of B2 materials: the Y-based B2 compounds (YAg, YCu, YIn, YRh, and YMg), the classic B2 alloys (NiAl, FeAl, AuCd, AuZn, CuZn, and AgMg), and the CsCl-type ionic compounds (CsCl, CsI, TlBr, and TlCl). In all the B2 materials, only YAg and YRh satisfy both necessary and su±cient conditions for enhanced ductility, while, like classic B2 alloys and ionic compounds, the YIn and YMg systems are predicted to be brittle, where the latter has been experimentally con¯rmed.
This general combined dislocation mechanics and DFT approach provides predictive maps for use in alloy design and understanding of anomalous ductility in B2 systems.Submitted by Sarah Shreeves ([email protected]) on 2013-03-07T20:50:47Z
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Analyzing Nation Branding Through Public Relations Lenses-- the Case of the 2008 Olympics Opening Ceremony
No full textAbstract
Studies across several disciplines, including marketing, political science and culture studies, have already touched on the concept of nation branding. Different experts with distinct academic backgrounds look at nation branding from different angles. This paper first takes a critical review of the conceptualizations of nation branding and then revisits the public relations theories, followed by a synthesis of public relations concepts with nation branding in a case study of the Beijing 2008 Opening Ceremony. It also seeks to open a conversation within the field of communication and public relations on the trend of nation branding. With an aim of contributing to the theoretical development of nation branding, the author attempts to review classic public relations theories and seeks for possibilities to integrate public relations theories into this fast-growing field.
Because nation branding is an umbrella term, the author narrows the research field to the case study of the Beijing Olympics Opening Ceremony. By comparatively analyzing the commentaries of the Opening Ceremony from CCTV and Australia Seven, this study attempts to to bring the Chinese nation branding events into the lenses of image management and communication
Low Intensity Conduction States in FeS₂: Implications for Absorption, Open-Circuit Voltage and Surface Recombination
No full textPyrite (FeS₂), being a promising material for future solar technologies, has so far exhibited in experiments an open-circuit voltage (OCV) of around 0.2 V, which is much lower than the frequently quoted 'accepted' value for the fundamental bandgap of ∼0.95 eV. Absorption experiments show large subgap absorption, commonly attributed to defects or structural disorder. However, computations using density functional theory with a semi-local functional predict that the bottom of the conduction band consists of a very low intensity sulfur p-band that may be easily overlooked in experiments because of the high intensity onset that appears 0.5 eV higher in energy. The intensity of absorption into the sulfur p-band is found to be of the same magnitude as contributions from defects and disorder. Our findings suggest the need to re-examine the value of the fundamental bandgap of pyrite presently in use in the literature. If the contribution from the p-band has so far been overlooked, the substantially lowered bandgap would partly explain the discrepancy with the OCV. Furthermore, we show that more states appear on the surface within the low energy sulfur p-band, which suggests a mechanism of thermalization into those states that would further prevent extracting electrons at higher energy levels through the surface. Finally, we speculate on whether misidentified states at the conduction band onset may be present in other materials
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