FY2010 - Oak Ridge National Laboratory

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<strong>Laboratory</strong>-Wide Fellowships— Weinberg Fellowship synthesis of superconductors, thus better positioning ORNL in the soliciting future industry participation and technology transfer through the DOE Office of Energy Efficiency and Renewable Energy Industrial Technologies Program (EERE-ITP) and future Office of Basic Energy Sciences (BES) initiatives. Results and Accomplishments In FY 2010, we developed the methods and apparatus as well as designed and conducted elevatedtemperature high-magnetic-field experiments that resulted in superconductors exhibiting up to a 59% increase in diamagnetic shielding. A high-frequency induction furnace capable of processing iron-based superconductors (SC) under high magnetic fields was designed and integrated into the ORNL 9T superconducting magnet, providing a unique capability at ORNL. Thus far we have magnetically annealed several iron-based superconductors with promising results indicated by magnetic susceptibility measurements. Initially, reaction sintered samples of LaFeAsO 0.90 F 0.10 and CeFeAsO 0.88 F 0.12 were annealed at 1300°C under a 9T field in an inert atmosphere. The magnetic susceptibility in both these samples decreased from -0.75 to -0.85 (4) (13% decrease) and -0.85 to -1.0 (4) (18% decrease), respectively. These results indicate that the advanced processing yields materials that are more “perfectly” diamagnetic. A recently discovered iron-based SC (Sr 4 V 2 O 6 Fe 2 As 2 ) was also magnetically annealed with less promising results that were linked to the nucleation of metastable phases during thermal processing. Additionally, synthesis was performed on two pressed pellets of constituents of NdFeAsO 0.88 F 0.12 . In order to establish a controlled baseline, both samples were pressed from the same batch of components and then reaction sintered either with or without a magnetic field. The orientations of the pellets within the superconducting solenoid magnet were held constant throughout the experiment. The Meissner effect below the critical temperature was enhanced in both the parallel and orthogonal to the field directions, and reductions in susceptibility from -0.17 to -0.27 (emu/g) or a 59% improvement were found in the parallel direction. In FY 2011 we plan a modified magnetic synthesis experiment on the NdFeAsO 0.88 F 0.12 superconductor. This experiment will include controlled rapid cooling rates to kinetically limit the formation of lower temperature structures that form during slow cooling. Additionally the microstructures of the LaFeAsO 0.90 F 0.10 , CeFeAsO 0.88 F 0.12 , and NdFeAsO 0.88 F 0.12 will be investigated by scanning electron microscopy (SEM) and energy filtered transmission electron microscopy (TEM) on selected areas of interest. Quantitative microscopy will be used to link the modified electronic and magnetic properties with the crystal structure, microstructure, and morphology. A beam time proposal will be submitted in FY 2011 to conduct neutron scattering powder diffraction experiments that will investigate the interrelation between the microstructure and the crystal/magnetic structural transformations through the superconducting transition temperature of materials reacted and annealed under extreme conditions. 268
<strong>Laboratory</strong>-Wide Fellowships— Weinberg Fellowship 05935 First-Principles Calculations and Computational Thermodynamic Modeling of Zn-S and Sn-S to Support Identifying Thermal Decomposition Pathways for Fabricating a New Photovoltaic Material, Cu 2 ZnSnS 4 Dongwon Shin Project Description Cu 2 ZnSnS 4 (CZTS) has recently gained great interest as an inexpensive candidate photovoltaic material; however, the complex chemistry of Cu-Zn-Sn-S makes the optimization of a high-efficiency CZTS synthesis process difficult. Computational thermodynamic modeling of Cu-In-Ga-Se played an important role in identifying thermodynamic decomposition pathways for the Cu(In , Ga)Se 2 -based photovoltaic devices production, and similar benefits are expected for CZTS. Current thermodynamic modeling for Cu-Zn-Sn-S is limited to Cu-Zn-Sn and Cu-S, but due to the high sulfur content of CZTS, thermodynamic modeling of Zn-S and Sn-S are necessary. Thermochemical measurements, such as heat capacities and formation enthalpies, directly affect the thermodynamic modeling quality and are thus preferred, but available data for Zn-S and Sn-S is only limited to phase equilibrium data. Evaluating thermodynamic parameters only with the phase boundary data may satisfy the relative Gibbs free energy among the phases to reproduce experimental phase boundaries, but they may be completely incorrect and hamper reliably extrapolating their energies to the higher order systems. First-principles calculations in this regard can provide thermochemical properties of sulfides to supplement scarce experimental data, and I propose a hybrid computational thermodynamic investigation, that is, a thermodynamic modeling and firstprinciples study on Zn-S and Sn-S. Mission Relevance Currently available photovoltaic materials are chalcogenide based and their usage of toxic (cadmium) or expensive (indium and tellurium) elements are projected to restrict the production of these solar cells. Thermodynamic modeling of Zn-S and Sn-S will eventually provide insight into the production of nontoxic and inexpensive new photovoltaic materials based on CZTS and will help garner new funding opportunities from DOE, such as EERE’s focus on solar energy technologies program. Results and Accomplishments The primary FY 2010 effort focused on the thermodynamic modeling of the Zn-S system and firstprinciples calculations on the binary sulfide phases in the Sn-S system. Gibbs free energy descriptions for the solid phases in Zn-S have been taken from the SGTE (Scientific Group Thermodata Europe) substance database, and that of the liquid phase has been evaluated to reproduce the experimental phase boundary data with an associates model. Total energies for tin sulfides have been obtained from firstprinciples calculations and used to evaluate formation enthalpies. First-principles thermochemical data will be used in the thermodynamic assessment to supplement scarce experimental data. 269
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NEUTRON SCIENCES ..................
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FY2010 - Oak Ridge National Laboratory

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