First-Principles Exploration of the Structural Motifs of Chalcogenides and Their Relationship to Electronic and Photovoltaic Properties

The bulk photovoltaic effect (BPVE) refers to the production of electric currents from the valence band promotion of electrons to the conduction band in a pure, undoped, and insulating material from the absorption of electromagnetic radiation. For a material to be able to create a current in response to non-polarized solar irradiation, it must be a polar compound with a band gap in the visible spectrum, 1.1 - 3.1 eV. The purpose of this work is to provide computational evidence that the pursuit of materials other than pure oxides for use as bulk photovoltaic materials is worthy. To convince the scientific community that compounds containing isoelectronic elements of O, the chalcogens S and Se, should be synthesized and, as importantly, are capable of being synthesized, three distinct subject areas will be presented. The first demonstrates that it is possible to alloy sulfur with an oxide perovskite, lead titanate (PTO), to create thermodynamically stable polar oxysulfides with band gaps in the visible spectrum. In the second, it will be shown that non-oxide non-perovskite materials can generate BPVE responses larger by an order of magnitude over the oxide perovskites listed in the literature to date. The third area does not address solar energy use directly. Instead, it alerts the scientist that creating ABS3 compounds by using temperature and time profiles used to synthesize ABO3 compounds has probably led to an incomplete characterization of these sulfides. As such, it provides first-principles based evidence that synthesis experimentation involving chalcogens does not and should not simply mirror that of pure oxide synthesis. The scientific community\u27s knowledge of chalcogenides is far from complete, opening up exciting possibilities for new material discoveries

Substantial optical dielectric enhancement by volume compression in LiAsSe2_2

Based on first-principles calculations, we predict a substantial increase in the optical dielectric function of LiAsSe$_2$ under pressure. We find that the optical dielectric constant is enhanced threefold under volume compression. This enhancement is mainly due to the dimerization strength reduction of the one-dimensional (1D) As--Se chains in LiAsSe$_2$, which significantly alters the wavefunction phase mismatch between two neighboring chains and changes the transition intensity. By developing a tight-binding model of the interacting 1D chains, the essential features of the low-energy electronic structure of LiAsSe$_2$ are captured. Our findings are important for understanding the fundamental physics of LiAsSe$_2$ and provide a feasible way to enhance the material optical response that can be applied to light harvesting for energy applications.Comment: 13 pages, 6 figure

Manned maneuvering unit applications for automated rendezvous and capture

Automated Rendezvous and Capture (AR&C) is an important technology to multiple National Aeronautics and Space Administration (NASA) programs and centers. The recent Johnson Spacecraft Center (JSC) AR&C Quality Function Deployment (QFD) has listed on-orbit demonstration of related technologies as a near term priority. Martin Marietta has been evaluating use of the Manned Maneuvering Unit (MMU) for a low cost near term on-orbit demonstration of AR&C technologies such as control algorithms, sensors, and processors as well as system level performance. The MMU Program began in 1979 as the method of repairing the Space Shuttle (STS) Thermal Protection System (the tiles). The units were not needed for this task, but were successfully employed during three Shuttle flights in 1984: a test flight was flown in in February as proof of concept, in April the MMU participated in the Solar Max Repair Mission, and in November the MMU's returned to space to successfully rescue the two errant satellites, Westar and Palapa. In the intervening years, the MMU simulator and MMU Qualification Test Unit (QTU) have been used for Astronaut training and experimental evaluations. The Extra-Vehicular Activities (EVA) Retriever has used the QTU, in an unmanned form, as a free-flyer on the Johnson Space Center (JSC) Precision Air Bearing Floor (PABF). Currently, the MMU is undergoing recertification for flight. The two flight units were removed from storage in September, 1991 and evaluation tests were performed. The tests demonstrated that the units are in good shape with no discrepancies that would preclude further use. The Return to Flight effort is currently clearing up recertification issues and evaluating the design against the present Shuttle environments

Estimating the funding gap of the Pension Benefit Guaranty Corporation

Pension Benefit Guaranty Corporation ; Pensions ; Insurance