Oak Ridge National Laboratory

About: Oak Ridge National Laboratory is a facility organization based out in Oak Ridge, Tennessee, United States. It is known for research contribution in the topics: Neutron & Ion. The organization has 31868 authors who have published 73724 publications receiving 2633689 citations. The organization is also known as: ORNL.

Polarization Control of Electron Tunneling into Ferroelectric Surfaces

Abstract: We demonstrate a highly reproducible control of local electron transport through a ferroelectric oxide via its spontaneous polarization. Electrons are injected from the tip of an atomic force microscope into a thin film of lead-zirconate titanate, Pb(Zr0.2Ti0.8)O3, in the regime of electron tunneling assisted by a high electric field (Fowler-Nordheim tunneling). The tunneling current exhibits a pronounced hysteresis with abrupt switching events that coincide, within experimental resolution, with the local switching of ferroelectric polarization. The large spontaneous polarization of the PZT film results in up to 500-fold amplification of the tunneling current upon ferroelectric switching. The magnitude of the effect is subject to electrostatic control via ferroelectric switching, suggesting possible applications in ultrahigh-density data storage and spintronics.

U.S. program on materials technology for ultra-supercritical coal power plants

Abstract: The efficiency of conventional fossil power plants is a strong function of the steam temperature and pressure. Research to increase both has been pursued worldwide, since the energy crisis in the 1970s. The need to reduce CO2 emissions has recently provided an additional incentive to increase efficiency. More recently, interest has been evinced in advanced combustion technologies utilizing oxygen instead of air for combustion. The main enabling technology in achieving the above goals is the development of stronger high temperature materials. Extensive research-and-development programs have resulted in numerous high-strength alloys for heavy section piping and for tubing needed to build boilers. The study reported on here is aimed at identifying, evaluating, and qualifying the materials needed for the construction of the critical components of coal-fired boilers that are capable of operating with steam at temperatures of 760 °C (1400 °F) and pressures of 35 MPa (5000 psi). The economic viability of such a plant has been explored. Candidate alloys applicable to various ranges of temperatures have been identified. Stress rupture tests have been completed on the base metal and on welds to a number of alloys. Steamside oxidation tests in an autoclave at 650 °C (1200 °F) and 800 °C (1475 °F) have been completed. Fireside corrosion tests have been conducted under conditions simulating those of waterwalls and superheater/reheater tubes. The weldability and fabricability of the alloys have been investigated. The capabilities of various overlay coatings and diffusion coatings have been examined. This article provides a status report on the progress achieved to date on this project.

A High Power Density Single-Phase PWM Rectifier With Active Ripple Energy Storage

Abstract: It is well known that single-phase pulse width modulation rectifiers have second-order harmonic currents and corresponding ripple voltages on the dc bus. The low-frequency harmonic current is normally filtered using a bulk capacitor in the bus, which results in low power density. However, pursuing high power density in converter design is a very important goal in the aerospace applications. This paper studies methods for reducing the energy storage capacitor for single-phase rectifiers. The minimum ripple energy storage requirement is derived independently of a specific topology. Based on the minimum ripple energy requirement, the feasibility of the active capacitor's reduction schemes is verified. Then, we propose a bidirectional buck-boost converter as the ripple energy storage circuit, which can effectively reduce the energy storage capacitance. The analysis and design are validated by simulation and experimental results.

Reduction of Fe(III) in sediments by sulphate-reducing bacteria

Abstract: REDUCTION of ferric iron (Fe(III)) to ferrous iron (Fe(II)) is one of the most important geochemical reactions in anaerobic aquatic sediments because of its many consequences for the organic and inorganic chemistry of these environments1. In marine environments, sulphate-reducing bacteria produce H2S, which can reduce iron oxyhydroxides2 to form iron sulphides. The presence of siderite (FeCO3) in marine sediments is anomalous, however, as it is unstable in the presence of H2S. Previous work3,4 has suggested a bacterial origin of siderite. Here we describe geochemical and microbiological studies which suggest that contemporary formation of siderite concretions in a salt-marsh sediment results from the activity of sulphate-reducing bacteria. We find that, instead of reducing Fe(III) indirectly through the production of sulphide, some of these bacteria can reduce Fe(III) directly through an enzymatic mechanism, producing siderite rather than iron sulphides. Sulphate-reducing bacteria may thus be an important and previously unrecognized agent for Fe(III) reduction in aquatic sediments and ground waters.

Influences of stress on the measurement of mechanical properties using nanoindentation: Part II. Finite element simulations

Abstract: The finite element method has been used to study the behavior of aluminum alloy 8009 during elastic-plastic indentation to establish how the indentation process is influenced by applied or residual stress. The study was motivated by the experiments of the preceding paper which show that nanoindentation data analysis procedures underestimate indentation contact areas and therefore overestimate hardness and elastic modulus in stressed specimens. The NIKE2D finite element code was used to simulate indentation contact by a rigid, conical indenter in a cylindrical specimen to which biaxial stresses were applied as boundary conditions. Indentation load-displacement curves were generated and analyzed according to standard methods for determining hardness and elastic modulus. The simulations show that the properties measured in this way are inaccurate because pileup is not accounted for in the contact area determination. When the proper contact area is used, the hardness and elastic modulus are not significantly affected by the applied stress.