Accelerated Cleanup Risk Reduction (open access)

Accelerated Cleanup Risk Reduction

There is no proven technology for remediating contaminant plume source regions in a heterogeneous subsurface. This project is an interdisciplinary effort to develop the requisite new technologies so that will be rapidly accepted by the remediation community. Our technology focus is hydrous pyrolysis/oxidation (HPO) which is a novel in situ thermal technique. We have expanded this core technology to leverage the action of steam injection and place an in situ microbial filter downstream to intercept and destroy the accelerated movement of contaminated groundwater. Most contaminant plume source regions, including the chlorinated solvent plume at LLNL, are in subsurface media characterized by a wide range in hydraulic conductivity. At LLNL, the main conduits for contaminant transport are buried stream channels composed of gravels and sands; these have a hydraulic conductivity in the range of 10{sup -1} to 10{sup -2} cm/s. Clay and silt units with a hydraulic conductivity of 10{sup -1} to 10{sup -6} cm/s bound these buried channels; these are barriers to groundwater movement and contain the highest contaminant concentrations in the source region. New remediation technologies are required because the current ones preferentially access the high conductivity units. HPO is an innovative process for the in situ destruction of …
Date: February 1, 1998
Creator: Knapp, R. B.; Aines, R. M.; Blake, R. G.; Copeland, A. B.; Newmark, R. L. & Tompson, A. F. B.
Object Type: Report
System: The UNT Digital Library
Advanced Lighting Program Development (BG9702800) Final Report (open access)

Advanced Lighting Program Development (BG9702800) Final Report

The report presents a long-range plan for a broad-based, coordinated research, development and market transformation program for reducing the lighting energy intensities in commercial and residential buildings in California without compromising lighting quality. An effective program to advance lighting energy efficiency in California must be based on an understanding that lighting is a mature field and the lighting industry has developed many specialized products that meet a wide variety of light needs for different building types. Above all else, the lighting field is diverse and there are applications for a wide range of lighting products, systems, and strategies. Given the range of existing lighting solutions, an effective energy efficient lighting research portfolio must be broad-based and diverse to match the diversity of the lighting market itself. The belief that there is one solution--a magic bullet, such as a better lamp, for example--that will propel lighting efficiency across all uses to new heights is, in the authors' opinion, an illusion. A multi-path program is the only effective means to raising lighting efficiency across all lighting applications in all building types. This report presents a list of 27 lighting technologies and concepts (key activities) that could form the basis of a coordinated …
Date: February 1, 1998
Creator: Rubinstein, Francis & Johnson, Steve
Object Type: Report
System: The UNT Digital Library
Advanced weigh-in-motion system for weighing vehicles at high speed (open access)

Advanced weigh-in-motion system for weighing vehicles at high speed

A state-of-the-art, Advanced Weigh-In-Motion (WIM) system has been designed, installed, and tested on the west bound side of Interstate I-75/I-40 near the Knox County Weigh Station. The project is a Cooperative Research and Development Agreement (CRADA) between Oak Ridge National Laboratory (ORNL) and International Road Dynamics, Inc. (IRD) sponsored by the Office of Uranium Programs, Facility and Technology Management Division of the Department of Energy under CRADA No. ORNL95-0364. ORNL, IRD, the Federal Highway Administration, the Tennessee Department of Safety and the Tennessee Department of Transportation have developed a National High Speed WIM Test Facility for test and evaluation of high-speed WIM systems. The WIM system under evaluation includes a Single Load Cell WIM scale system supplied and installed by IRD. ORNL developed a stand-alone, custom data acquisition system, which acquires the raw signals from IRD`s in-ground single load cell transducers. Under a separate contract with the Federal Highway Administration, ORNL designed and constructed a laboratory scale house for data collection, analysis and algorithm development. An initial advanced weight-determining algorithm has been developed. The new advanced WIM system provides improved accuracy and can reduce overall system variability by up to 30% over the existing high accuracy commercial WIM system.
Date: February 1, 1998
Creator: Beshears, D.L.; Muhs, J.D. & Scudiere, M.B.
Object Type: Report
System: The UNT Digital Library
Advances in the operation of the DIII-D neutral beam computer systems (open access)

Advances in the operation of the DIII-D neutral beam computer systems

The DIII-D neutral beam system routinely provides up to 20 MW of deuterium neutral beam heating in support of experiments on the DIII-D tokamak, and is a critical part of the DIII-D physics experimental program. The four computer systems previously used to control neutral beam operation and data acquisition were designed and implemented in the late 1970`s and used on DIII and DIII-D from 1981--1996. By comparison to modern standards, they had become expensive to maintain, slow and cumbersome, making it difficult to implement improvements. Most critical of all, they were not networked computers. During the 1997 experimental campaign, these systems were replaced with new Unix compliant hardware and, for the most part, commercially available software. This paper describes operational experience with the new neutral beam computer systems, and new advances made possible by using features not previously available. These include retention and access to historical data, an asynchronously fired ``rules`` base, and a relatively straightforward programming interface. Methods and principles for extending the availability of data beyond the scope of the operator consoles will be discussed.
Date: February 1, 1998
Creator: Phillips, J. C.; Busath, J. L.; Penaflor, B. G.; Piglowski, D.; Kellman, D. H.; Chiu, H. K. et al.
Object Type: Article
System: The UNT Digital Library
Los Alamos opacity web page (open access)

Los Alamos opacity web page

The Los Alamos opacity data base is now available on the World Wide Web at http://t4.lanl.gov. The data base contains both the original Astrophysical Opacity Library distributed worldwide in the 1980`s (for historical reference) and the new improved opacities from the Light Element Detailed Configuration OPacity (LEDCOP) code. Users can access the opacity data using the multigroup opacity code TOPS to obtain Rosseland and Planck gray opacities, group mean opacities over selected energy ranges, the monochromatic absorption coefficients and the average ionization over a wide range of temperatures and densities. As described in this paper, these quantities are available for all of the elements presently on the data base and TOPS will provide the same quantities for any arbitrary mixture of these elements.
Date: February 1, 1998
Creator: Magee, N.H. Jr. & Clark, R.E.H.
Object Type: Article
System: The UNT Digital Library
An alternative test for verifying electronic balance linearity (open access)

An alternative test for verifying electronic balance linearity

This paper presents an alternative method for verifying electronic balance linearity and accuracy. This method is being developed for safeguards weighings (weighings for the control and accountability of nuclear material) at the Idaho National Engineering and Environmental Laboratory (INEEL). With regard to balance linearity and accuracy, DOE Order 5633.3B, Control and Accountability of Nuclear Materials, Paragraph 2, 4, e, (1), (a) Scales and Balances Program, states: ``All scales and balances used for accountability purposes shall be maintained in good working condition, recalibrated according to an established schedule, and checked for accuracy and linearity on each day that the scale or balance is used for accountability purposes.`` Various tests have been proposed for testing accuracy and linearity. In the 1991 Measurement Science Conference, Dr. Walter E. Kupper presented a paper entitled: ``Validation of High Accuracy Weighing Equipment.`` Dr. Kupper emphasized that tolerance checks for calibrated, state-of-the-art electronic equipment need not be complicated, and he presented four easy steps for verifying that a calibrated balance is operating correctly. These tests evaluate the standard deviation of successive weighings (of the same load), the off-center error, the calibration error, and the error due to nonlinearity. This method of balance validation is undoubtedly an authoritative …
Date: February 1, 1998
Creator: Thomas, I. R.
Object Type: Article
System: The UNT Digital Library
The Aluminum Stabilized Conductor for the Fermilab D-Zero Solenoid (open access)

The Aluminum Stabilized Conductor for the Fermilab D-Zero Solenoid

This report addresses the aluminum stabilized conductor for the Fermilab D0 Solenoid.
Date: February 1, 1998
Creator: Smith, R. P.
Object Type: Article
System: The UNT Digital Library
Analyses of interfacial shear debonding in fiber-reinforced ceramic composites (open access)

Analyses of interfacial shear debonding in fiber-reinforced ceramic composites

An important toughening mechanism in fiber-reinforced ceramic composites is pullout of fibers from the matrix during matrix cracking. This relies on mode II (i.e., shear) debonding at the fiber/matrix interface which can be analyzed using either the strength-based or the energy-based criterion. In the strength-based approach, debonding occurs when the maximum interfacial shear stress induced by the applied load reaches the interfacial shear strength, {tau}{sub s}. In the energy-based approach, a mode II crack propagating along the interface is considered, and debonding occurs when the energy release rate due to crack propagation reaches the interface debond energy, {Gamma}{sub 1}. Based on the above two criteria, the applied stress on the fiber to initiate debonding (i.e., the initial debond stress), {sigma}{sub d}, can be derived. The first issue considered in the present study is the relation between {tau}{sub s} and {Gamma}{sub 1}. Also, for a monolithic ceramic, the tensile strength can be related to its defect size based on the Griffith theory. A question is hence raised as to whether the initial debond stress for fiber pullout in a fiber-reinforced ceramic composite can be related to any defect at the interface.
Date: February 1, 1998
Creator: Hsueh, C.H. & Becher, P.F.
Object Type: Article
System: The UNT Digital Library
Analysis of Source Spectra, Attenuation, and Site Effects From Central and Eastern United States Earthquakes (open access)

Analysis of Source Spectra, Attenuation, and Site Effects From Central and Eastern United States Earthquakes

This report describes the results from three studies of source spectra, attenuation, and site effects of central and eastern United States earthquakes. In the first study source parameter estimates taken from 27 previous studies were combined to test the assumption that the earthquake stress drop is roughly a constant, independent of earthquake size. 200 estimates of stress drop and seismic moment from eastern North American earthquakes were combined. It was found that the estimated stress drop from the 27 studies increases approximately as the square-root of the seismic moment, from about 3 bars at 10{sup 20} dyne-cm to 690 bars at 10{sup 25} dyne-cm. These results do not support the assumption of a constant stress drop when estimating ground motion parameters from eastern North American earthquakes. In the second study, broadband seismograms recorded by the United States National Seismograph Network and cooperating stations have been analysed to determine Q{sub Lg} as a function of frequency in five regions: the northeastern US, southeastern US, central US, northern Basin and Range, and California and western Nevada. In the third study, using spectral analysis, estimates have been made for the anelastic attenuation of four regional phases, and estimates have been made for the …
Date: February 1, 1998
Creator: Lindley, G.
Object Type: Report
System: The UNT Digital Library
Los Angeles Department of Water and Power Electric and Hybrid Vehicle Program site operator program (open access)

Los Angeles Department of Water and Power Electric and Hybrid Vehicle Program site operator program

During the term of the above mentioned agreement, the Los Angeles Department of Water and Power (LADWP), a municipal utility serving the citizens of Los Angeles, marked its tenth year of involvement in testing and promoting electric vehicles as part of Los Angeles` overall air quality improvement program, and as a means of improving the regions` economic competitiveness through the creation of new industries. LADWP maintained and operated twenty electric vehicles (EVs) during the test period. These vehicles consisted of six G-Vans, four Chrysler TEVans, five U.S. Electricar pickup trucks, and five U.S. Electricar Prizms. LADWP`s electric transportation program also included infrastructure, public transit development, public and awareness, and legislative and regulatory activities.
Date: February 1, 1998
Creator: unknown
Object Type: Report
System: The UNT Digital Library
Anisotropic behavior in 0.5m scale blocks of Topopah Spring tuff (open access)

Anisotropic behavior in 0.5m scale blocks of Topopah Spring tuff

Laboratory tests on 0.5 meter scale blocks of Topopah Spring tuff were performed to determine fluid flow and mechanical behavior of samples containing fractures. Results include data for a comprehensive set of flow measurements through a rock sample containing a horizontally oriented fracture at uniaxial stress conditions up to 8 MPa at room temperature. Directional channeling, rather than mean fracture aperture, controls the flow. On the time scale of these experiments, inhibition is negligible.
Date: February 1, 1998
Creator: Blair, S. C. & Costantino, M. S.
Object Type: Article
System: The UNT Digital Library
Anisotropies in the cosmic microwave background: Theory (open access)

Anisotropies in the cosmic microwave background: Theory

Anisotropies in the Cosmic Microwave Background (CMB) contain a wealth of information about the past history of the universe and the present values of cosmological parameters. I online some of the theoretical advances of the last few years. In particular, I emphasize that for a wide class of cosmological models, theorists can accurately calculate the spectrum to better than a percent. The spectrum of anisotropies today is directly related to the pattern of inhomogeneities present at the time of recombination. This recognition leads to a powerful argument that will enable us to distinguish inflationary models from other models of structure formation. If the inflationary models turn out to be correct, the free parameters in these models will be determined to unprecedented accuracy by the upcoming satellite missions.
Date: February 1, 1998
Creator: Dodelson, S.
Object Type: Article
System: The UNT Digital Library
An APFIM and TEM study of Ni{sub 4}Mo precipitation in a commercial Ni-28% Mo-1.4% Fe-0.4% Cr wt. % alloy (open access)

An APFIM and TEM study of Ni{sub 4}Mo precipitation in a commercial Ni-28% Mo-1.4% Fe-0.4% Cr wt. % alloy

Ni-Mo alloys containing at least 26 wt.% Mo have a negligible corrosion rate in boiling 10% hydrochloric acid and are therefore used in corrosive environments. A series of commercial Ni-Mo alloys has been developed with subtle variations in chemical composition. These alloys usually contain {approximately} 28 wt.% Mo with additions of up to 5% Fe and Cr. A significant amount of research has been performed to understand the microstructure and properties of these alloys, although most of the effort has concentrated on the Ni-Mo binary system. In some alloys with low Fe and Cr contents, a severe embrittlement problem has been observed due to the formation of the Ni{sub 4}Mo (D1{sub a}-ordered) phase within the microstructure. This research focuses on a commercial alloy with nominal composition Ni-28% Mo-1.4% Fe-0.4% Cr-0.1% Mn-0.003 wt.% C. The material supplied was a heat treatment coupon which had been attached to a large vessel during fabrication. Assessment of the chemical analysis of the alloy suggested that detrimental phases could be present or might appear during subsequent repair work. Therefore, it was important to assess the microstructural condition of the vessel, and in particular the kinetics of precipitation of Ni{sub 4}Mo.
Date: February 1, 1998
Creator: Thomson, R. C.; Brown, N.; Bates, J. S.; Russell, K. F. & Miller, M. K.
Object Type: Report
System: The UNT Digital Library
Application of equivalent elastic methods in three-dimensional finite element structural analysis (open access)

Application of equivalent elastic methods in three-dimensional finite element structural analysis

This paper describes use of equivalent solid (EQS) modeling to obtain efficient solutions to perforated material problems using three-dimensional finite element analysis (3D-FEA) programs. It is shown that the accuracy of EQS methods in 3D-FEA depends on providing sufficient equivalent elastic properties to allow the EQS material to respond according to the elastic symmetry of the pattern. Peak stresses and ligament stresses are calculated from the EQS stresses by an appropriate 3D-FEA submodel approach. The method is demonstrated on the problem of a transversely pressurized simply supported plate with a central divider lane separating two perforated regions with circular penetrations arranged in a square pattern. A 3D-FEA solution for a model that incorporates each penetration explicitly is used for comparison with results from an EQS solution for the plate. Results for deflection and stresses from the EQS solution are within 3% of results from the explicit 3D-FE model. A solution to the sample problem is also provided using the procedures in the ASME B and PV Code. The ASME B and PV Code formulas for plate deflection were shown to overestimate the stiffening effects of the divider lane and the outer stiffening ring.
Date: February 1, 1998
Creator: Jones, D.P.; Gordon, J.L.; Hutula, D.N.; Holliday, J.E. & Jandrasits, W.G.
Object Type: Article
System: The UNT Digital Library
Application of RELAP5/MOD3.1 code to the LOFT test L3-6 (open access)

Application of RELAP5/MOD3.1 code to the LOFT test L3-6

A calculation of LOFT Experiment L3-6, a small break equivalent to a 4-in diameter rupture in the cold leg of a four-loop commercial pressurized water reactor, has been performed to help validate RELAP5/MOD3.1 for this application. The version of the code to be used is SCDAP/RELAP5/MOD3.1.8d0. Three calculations were carried out in order to study the sensitivity to change break nozzle superheated discharge coefficient. Conducted comparative analysis of the LOFT L3-6 experiment shows on the whole a reasonable agreement between calculated data. Some discrepancies in the system pressure do not distort a picture of the transient. 6 refs.
Date: February 1, 1998
Creator: Pylev, S. S. & Roginskaja, V. L.
Object Type: Report
System: The UNT Digital Library
Application of sensitivity coefficients for heat conduction problems (open access)

Application of sensitivity coefficients for heat conduction problems

In parameter estimation considerable insight is provided by examining sensitivity coefficients. This paper focuses on the use of sensitivity coefficients in connection with estimating thermal properties in the heat conduction equation. A general methodology for computing sensitivity coefficients can be an important design tool. The use of such a tool is demonstrated in this paper. A control volume, finite element program is used, and briefly described, to implement numerical sensitivity coefficient calculations. In this approach general problems can be studied. Several example problems are presented to demonstrate the insight gained from sensitivity coefficients. The problems are selected from experimental studies to characterize the thermal properties of carbon-carbon composite. Sensitivity coefficients show that in an experiment that is not well designed, additional materials in the experimental configuration can have a larger impact on the temperature than the material of interest. Two-dimensional configurations demonstrate that there can be isolated areas of insensitivity and the difficulty of estimating multiple parameters.
Date: February 1, 1998
Creator: Dowding, K. J.; Blackwell, B. F. & Cochran, R. J.
Object Type: Article
System: The UNT Digital Library
Applying Macro Design Tools to the Design of MEMS Accelerometers (open access)

Applying Macro Design Tools to the Design of MEMS Accelerometers

This paper describes the design of two different surface micromachined (MEMS) accelerometers and the use of design and analysis tools intended for macro sized devices. This work leverages a process for integrating both the micromechanical structures and microelectronics circuitry of a MEMS accelerometer on the same chip. In this process, the mechanical components of the sensor are first fabricated at the bottom of a trench etched into the wafer substrate. The trench is then filled with oxide and sealed to protect the mechanical components during subsequent microelectronics processing. The wafer surface is then planarized in preparation for CMOS processing. Next, the CMOS electronics are fabricated and the mechanical structures are released. The mechanical structure of each sensor consists of two polysilicon plate masses suspended by multiple springs (cantilevered beam structures) over corresponding polysilicon plates fixed to the substrate to form two parallel plate capacitors. One polysilicon plate mass is suspended using compliant springs forming a variable capacitor. The other polysilicon plate mass is suspended using very stiff springs acting as a fixed capacitor. Acceleration is measured by comparing the variable capacitance with the fixed capacitance during acceleration.
Date: February 1, 1998
Creator: Davies, B. R.; Rodgers, M. S. & Montague, S.
Object Type: Article
System: The UNT Digital Library
Assessing quality management in an R and D environment (open access)

Assessing quality management in an R and D environment

Los Alamos National Laboratory (LANL) is a premier research and development institution operated by the University of California for the US Department of Energy. Since 1991, LANL has pursued a heightened commitment to developing world-class quality in management and operations. In 1994 LANL adopted the Malcolm Baldrige National Quality Award criteria as a framework for all activities and initiated more formalized customer focus and quality management. Five measurement systems drive the current integration of quality efforts: an annual Baldrige-based assessment, a customer focus program, customer-driven performance measurement, an employee performance management system and annual employee surveys, and integrated planning processes with associated goals and measures.
Date: February 1, 1998
Creator: Thompson, B.D.
Object Type: Article
System: The UNT Digital Library
The atom-probe field ion microscope: Applications in surface science (open access)

The atom-probe field ion microscope: Applications in surface science

The ability to locate an individual atom on a surface, remove it in a controlled fashion, and determine its chemical identity makes the atom-probe field-ion microscope an extremely powerful tool for the analysis of solid surfaces. By itself, the field ion microscope has contributed significantly to the understanding of surface atomic structure, single-atom surface diffusion, and the detailed interactions that occur between atoms and defects on surfaces. When used in combination with the atom-probe mass spectrometer there have been several additional areas within the traditional definition of ``surface science`` where the chemical identification capability of the atom probe has led to new insights. In this paper these applications are reviewed focusing on two specific areas: surface segregation in intermetallic alloys and chemical reactions on metal surfaces.
Date: February 1, 1998
Creator: Kellogg, G. L.
Object Type: Article
System: The UNT Digital Library
Atom probe field ion microscopy of high resistivity materials (open access)

Atom probe field ion microscopy of high resistivity materials

Over the last 30 years the atom probe has proved to be a powerful tool for studying nanometer-sized compositional fluctuations in a wide range of metallic alloys but has had only limited applications to semiconductors and ceramics. One of the primary reasons for this difference is the higher resistivity of semiconducting and ceramic specimens. Because of this high resistivity, the high voltage field evaporation pulse is attenuated before it reaches the apex of the specimen thereby making the pulse ineffective for field evaporation. Experiments have demonstrated that both variants of the voltage-pulsed atom probe (i.e., those instruments in which the field evaporation pulse is applied directly to the specimen and those in which the negative pulse is applied to a counter electrode in front of the specimen) are equally affected. In this overview, the limits of applicability of the voltage-pulsed atom probe to high resistivity materials are examined. In this study, a wide range of materials have been examined to determine whether field ion microscopy and voltage-pulsed field evaporation can be achieved and the results are summarized in the report. Field ion microscopy including dc field evaporation was possible for all materials except bulk ceramic insulators and glasses. Field ion …
Date: February 1, 1998
Creator: Sibrandij, S. J.; Larson, D. J. & Miller, M. K.
Object Type: Article
System: The UNT Digital Library
Atom probe field ion microscopy of polysynthetically twinned titanium aluminide (open access)

Atom probe field ion microscopy of polysynthetically twinned titanium aluminide

Two phase {gamma}-based TiAl alloys are attractive for structural applications at high temperatures because they possess good elevated-temperature mechanical properties, low density, and good creep and oxidation resistance. The microstructures of these alloys consist of plates of the near equiatomic {gamma} phase (L1{sub 0}-ordered structure) and the Ti{sub 3}Al {alpha}{sub 2} phase (D0{sub 19}-ordered structure). It is of great interest to study the details of the lamellar {alpha}{sub 2} + {gamma} microstructure because the interface stability is the key to providing a usable high temperature material. Polysynthetically twinned (PST) TiAl crystals have been developed in order to systematically study the lamellar microstructure. These PST materials contain no high angle grain boundaries and have an single set of aligned lamellar of {alpha}{sub 2} and {gamma} phases. Therefore, PST samples facilitate the study of the dependence of mechanical properties on lamellar structure by providing a known, consistent set of aligned lamellar. Previous transmission electron microscopy studies of PST TiAl have shown that Cr and Mo segregation occurs at certain non-coherent {gamma}/{gamma} twin boundaries. These studies also found a depletion of aluminum at certain {gamma}/{gamma} interfaces, showing ``{alpha}{sub 2}-like`` compositions. However, no evidence was found of even a few unit cells of the …
Date: February 1, 1998
Creator: Larson, D. L.; Miller, M. K.; Inui, H. & Yamaguchi, M.
Object Type: Report
System: The UNT Digital Library
The atomic-scale origins of grain boundary superconducting properties (open access)

The atomic-scale origins of grain boundary superconducting properties

Due to the extremely short coherence lengths of the high-{Tc} superconductors, defects such as grain boundaries are obvious barriers to the flow of supercurrent. Within a few months of the discovery of these materials, it was shown how the critical current dropped four orders of magnitude as the grain boundary misorientation increased from zero to 45{degree}. Even today, there is no quantitative understanding of this behavior. A qualitative understanding is however possible through atomic resolution Z-contrast imaging on YBa{sub 2}Cu{sub 3}O{sub 7{minus}{delta}} and SrTiO{sub 3} bicrystal grain boundaries, combined with bond-valence-sum analysis. The Z-contrast image of a YBa{sub 2}Cu{sub 3}O{sub 7{minus}{delta}} low angle grain boundary shows the same kind of reconstructed dislocation cores as seen in SrTiO{sub 3}, containing reconstructions on both the Cu and Y/Ba sublattices. An image of an asymmetric 30{degree} boundary in YBa{sub 2}Cu{sub 3}O{sub 7{minus}{delta}} shows the same units and unit sequence as expected for SrTiO{sub 3}. YBa{sub 2}Cu{sub 3}O{sub 7{minus}{delta}} boundaries are wavy because of their non-equilibrium growth process, and therefore mostly asymmetric in nature, although small segments have the symmetric structure. It seems reasonable to assume that boundaries of other angles will also have similar structures in these two materials.
Date: February 1, 1998
Creator: Pennycook, S. J.; Chisholm, M. F.; Buban, J.; Browning, N. D.; Prouteau, C. & Nellist, P. D.
Object Type: Article
System: The UNT Digital Library
Atomic-scale structures in complex solids by Z-contrast STEM and first-principles theory (open access)

Atomic-scale structures in complex solids by Z-contrast STEM and first-principles theory

Modern high-resolution scanning transmission electron microscopes are capable of forming electron probes below 2{angstrom}, sufficiently small to allow an atomic resolution Z-contrast image to be formed from many materials. Such images, formed with a high angle annular detector, are incoherent in nature, and can be described as a convolution of the probe intensity profile with an object function peaked sharply at the atomic sites. Unlike phase contrast microscopy, there is no phase problem associated with an incoherent image, and direct inversion to the projected atomic structure is possible. A quantitative method for structure retrieval is maximum entropy analysis, although under Scherzer incoherent conditions the probe tails are small, and peaks in the image intensity correspond closely to atomic positions. In such cases, an intuitive structure determination may be possible. Electron energy loss spectroscopy (EELS) may also be performed simultaneously, using the Z-contrast image to position the probe over selected atomic columns, providing complementary information on composition and electronic structure. On the theoretical side, present-day computers are now capable of electronic structure calculations that can be used to determine the preferred atomic arrangements in complex systems. Both first-principles and semiempirical approaches have been developed with complementary capabilities. Here the authors present …
Date: February 1, 1998
Creator: Pennycook, S.J.; Pantelides, S.T.; Maiti, A.; Chisholm, M.F. & Yan, Y.
Object Type: Article
System: The UNT Digital Library
Atomic structure of a polar ceramic/metal interface: {l_brace}222{r_brace}MgO/Cu (open access)

Atomic structure of a polar ceramic/metal interface: {l_brace}222{r_brace}MgO/Cu

{l_brace}222{r_brace}MgO/Cu is one of the most extensively characterized ceramic/metal interfaces, in view of the atom probe field ion microscopy, Z-contrast Scanning Transmission Electron Microscopy (STEM), and spatially resolved Electron energy loss spectroscopy (EELS) measurements performed by the present authors, as well as the high resolution electron microscopy (HREM) of this system by others. Atomistic simulations with local density functional theory (LDFT) and Molecular Dynamics (MD) have been performed to gain additional insight into the structure of this interface. This presentation describes an interface interatomic potential for {l_brace}222{r_brace}MgO/Cu derived from LDFT total energy calculations, and its application to structural properties, including the terminating species, the absence of dislocation standoff, and the symmetry of the interfacial dislocation network.
Date: February 1, 1998
Creator: Benedek, R.; Shashkov, D. A.; Seidman, D. N.; Muller, D. A.; Silcox, J.; Chisholm, M. F. et al.
Object Type: Article
System: The UNT Digital Library