WorldCat Identities

Wall, M. A.

Overview
Works: 12 works in 13 publications in 1 language and 53 library holdings
Roles: Author
Publication Timeline
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Most widely held works by M. A Wall
Liquid crystals for nondestructive testing by M. A Wall( Book )

2 editions published in 1972 in English and held by 2 WorldCat member libraries worldwide

ISOTHERMAL (DELTA)/(ALPHA-PRIME) TRANSFORMATION AND TTT DIAGRAM IN A PLUTONIUM GALLIUM ALLOY( )

1 edition published in 2005 in English and held by 0 WorldCat member libraries worldwide

Differential scanning calorimetry (DSC) is used as an alternative approach to determining the tine-temperature-transformation (TTT) diagram for the martensitic delta to alpha-prime transformation in a Pu-2.0 at% Ga alloy. Previous work suggests that the TTT diagram for a similar alloy exhibits an unusual double-C curve for isothermal holds of less than 100 minutes. Here, we extend this diagram to 18 hours, and confirm the double-C curve behavior. When the sample is cooled prior to the isothermal holds, the delta to alpha-prime transformation is observed as several overlapping exothermic peaks. These peaks are very reproducible, and they are believed to be the result of different kinds of delta to alpha-prime martensitic transformation. This may be due to the presence of different nucleation sites and/or different morphologies
Advanced Transmission Electron Microscopy of Pu Alloys( )

1 edition published in 2003 in English and held by 0 WorldCat member libraries worldwide

The characterization of microstructural changes in Pu-Ga alloys resulting from storage and aging phenomena is an important technical challenge to the nuclear Stockpile Stewardship program. We have identified at least two age-related phenomena that may occur in Pu alloys, dimensional changes due to the initial transient, helium accumulation, and void swelling, and phase instability. The initial transient is a well-known effect that results from the initial cascade damage. This form of dimensional change tends to saturate within approximately two years. A second contributor to dimensional change is the build-up of helium as a result of the alpha decay. Helium is generated at a rate of approximately 40 parts per million per year. Positron annihilation results by Howell indicate that the helium atoms will quickly fill a nearby vacancy and diffuse through the lattice as a helium filled vacancy. Void swelling is potentially the most severe mechanism of dimensional change in Pu alloys. It has been observed in all-materials exposed to irradiation, but has yet to be seen in naturally aged Pu. Phase instability is a potential concern due to the fact that the {delta}-phase is thermodynamically metastable at room temperature. Timofeeva has shown that the {delta}-phase will decompose to {delta}-phase and Pu{sub 3}Ga given enough time at ambient temperature. At sub-ambient temperatures, the {delta}-phase undergoes a displacive or martensitic phase transformation to the monoclinic {alpha}{prime}-phase, which is approximately 20% more dense. Phase transformations such as these would result in density changes, dimensional changes, and Changes in mechanical properties. Traditional characterization techniques such as optical microscopy, x-ray diffraction and scanning electron microscopy are insensitive to many of the age-related microstructural changes. In this investigation, we have applied advanced transmission electron microscopy (TEM) to investigate the microstructure and bonding of Pu alloys. A 300 keV Phillips CM300FEG with a field emission gun electron source and Gatan Imaging Filter are used for the investigations
Capabilities for Testing the Electronic Configuration in Pu( )

1 edition published in 2006 in English and held by 0 WorldCat member libraries worldwide

The benchmarking of theoretical modeling is crucial to the ultimate determination of the nature of the electronic structure of Pu. Examples of experimental techniques used for cross checking state of the art calculations will be given
Manufacturing Ultra-Precision Meso-Scale Targets by Coining Summary Report( )

1 edition published in 2004 in English and held by 0 WorldCat member libraries worldwide

There exists a certain type of high energy density physics experiment that requires a very precise, one or two-dimensional sinusoidal pattern to be imprinted on the surface of a thin disc-shaped specimen. Early discussions of potential fabrication processes included precision lapping using a substrate with the sinusoidal patterns, diamond turning, and various vapor deposition techniques. The process of coining, in which the sinusoidal pattern is pressed into the surface of a flat disc, was also proposed. Extensive discussions indicated that the low cost of coining and likelihood of success warranted this proof-of-principal investigation
Utilizing Nano-focussed Bremstrahlung Isochromat Spectroscopy (nBIS) to Determine the Unoccupied Electronic Structure of Pu( )

1 edition published in 2005 in English and held by 0 WorldCat member libraries worldwide

Understanding the behavior of 5f electrons remains an unrealized ambition of condensed matter physics [1,2]. Recently, there has been a large amount of interest in the actinides, particularly plutonium, driven by the complex and intriguing behavior of Pu and several of its compounds [3-5]. This has prompted both theoretical and experimental investigations of 5f metals and compounds. Of the different allotropes of Pu, the d-phase is of particular interest because of the high symmetry crystal structure and the stability of the phase to low temperatures when alloyed with small amounts of trivalent elements. Consequently much of the recent experimental and theoretical work has focused on this allotrope. From an experimental point of view, the reactivity and radioactivity of Pu, and the complexity of the phase diagram, make it exceedingly complicated to collect high-quality data. Investigations of these complex behaviors all point back to being caused by the intriguing interplay of the various electron states and in particular the behavior of the 5f states. While there are a number of ongoing experimental efforts directed at determining the occupied electronic structure of Pu, there is essentially no experimental data on the unoccupied electronic structure of Pu. We aim to determine the conduction band (unoccupied) electronic structure of Pu and other actinides in a phase specific fashion and emphasizing bulk contributions by using Nano-focussed Bremstrahlung Isochromat Spectroscopy (nBIS). Bremstrahlung Isochromat Spectroscopy (BIS) is the high-energy variant of inverse photoelectron spectroscopy (IPES: electron in, photon out), which is essentially the time reversal of photoelectron spectroscopy (photon in, electron out). IPES can be used to follow the dispersion of electronic states in ordered samples. Owing to its low energies, IPES is usually very surface sensitive. However, by working at higher energies (>200 eV), we will sample preferentially for bulk properties, downgrading the impact of surface effects. Thus, from BIS, we would have a direct measure of the conduction band or unoccupied electronic structure of the bulk Pu
Experimental Benchmarking of Pu Electronic Structure( )

1 edition published in 2005 in English and held by 0 WorldCat member libraries worldwide

The standard method to determine the band structure of a condensed phase material is to (1) obtain a single crystal with a well defined surface and (2) map the bands with angle resolved photoelectron spectroscopy (occupied or valence bands) and inverse photoelectron spectroscopy (unoccupied or conduction bands). Unfortunately, in the case of Pu, the single crystals of Pu are either nonexistent, very small and/or having poorly defined surfaces. Furthermore, effects such as electron correlation and a large spin-orbit splitting in the 5f states have further complicated the situation. Thus, we have embarked upon the utilization of unorthodox electron spectroscopies, to circumvent the problems caused by the absence of large single crystals of Pu with well-defined surfaces. Our approach includes the techniques of resonant photoelectron spectroscopy [1], x-ray absorption spectroscopy [1,2,3,4], electron energy loss spectroscopy [2,3,4], Fano Effect measurements [5], and Bremstrahlung Isochromat Spectroscopy [6], including the utilization of micro-focused beams to probe single-crystallite regions of polycrystalline Pu samples. [2,3,6]
Dual Beam FIB for Imaging, Nano-Sectioning and Sample Preparation of Spores : Initial Results( )

1 edition published in 2004 in English and held by 0 WorldCat member libraries worldwide

Results from the first use of Focused Ion Beam (FIB) technology to section Bacillus spores at LLNL in a dual-beam (electron and ion) instrument is presented and discussed. With the use of a dual-beam instrument, high resolution imaging of single spores using low voltage scanning electron microscopy followed by FIB sectioning, SEM imaging of internal structure of the same spore is demonstrated to be possible. Additionally, FIB is shown to be able to precisely micro-machine spores thus potentially facilitating micro-scale experiments on single spores
L1 Report for the Enhanced Surveillance Campaign Experimental Benchmarking of Pu Electronic Structure( )

1 edition published in 2006 in English and held by 0 WorldCat member libraries worldwide

The objective of this work is to develop and/or apply advanced diagnostics to the understanding of aging of Pu. Advanced characterization techniques such as photoelectron and x-ray absorption spectroscopy will provide fundamental data on the electronic structure of Pu phases. These data are crucial for the validation of the electronic structure methods. The fundamental goal of this project is to narrow the parameter space for the theoretical modeling of Pu aging. The short-term goal is to perform experiments to validate electronic structure calculations of Pu. The long-term goal is to determine the effects of aging upon the electronic structure of Pu. Many of the input parameters for aging models are not directly measurable. These parameters will need to be calculated or estimated. Thus a First Principles-Approach Theory is needed, but it is unclear what terms are important in the Hamiltonian (H{Psi} = E{Psi}). Therefore, experimental data concerning the 5f electronic structure are needed, to determine which terms in the Hamiltonian are important. The data obtained in this task are crucial for reducing the uncertainty of Task LL-01-developed models and predictions. The data impact the validation of electronic structure methods, the calculation of defect properties, the evaluation of helium diffusion, and the validation of void nucleation models. The importance of these activities increases if difficulties develop with the accelerating aging alloy approach
Measurement and Modeling of Resistivity as a Microscale Tool to Quantify the Volume Fraction of Lenticular (alpha)' Particles in a Partially Transformed (delta)-phase Pu-Ga Matrix( )

1 edition published in 2005 in English and held by 0 WorldCat member libraries worldwide

We have measured and modeled the change in electrical resistivity due to partial transformation to the martensitic {alpha}{prime}-phase in a {delta}-phase Pu-Ga matrix. The primary objective is to relate the change in resistance, measured with a 4-probe technique during the transformation, to the volume fraction of the {alpha}{prime} phase created in the microstructure. Analysis by finite element methods suggests that considerable differences in the resistivity may be anticipated depending on the orientational and morphological configurations of the {alpha}{prime} particles. Finite element analysis of the computed resistance of an assembly of lenticular shaped particles indicates that series resistor or parallel resistor approximations are inaccurate and can lead to an underestimation of the predicted amount of {alpha}{prime} in the sample by 15% or more. Comparison of the resistivity of a simulated network of partially transformed grains or portions of grains suggests that a correction to the measured resistivity allows quantification of the amount of {alpha}{prime} phase in the microstructure with minimal consideration of how the {alpha}{prime} morphology may evolve. It is found that the average of the series and parallel resistor approximations provide the most accurate relationship between the measured resistivity and the amount of {alpha}{prime} phase. The methods described here are applicable to any evolving two-phase microstructure in which the resistance difference between the two phases is measurable
Metastability and Delta-Phase Retention in Plutonium Alloys Final Report of LDRD Project 01-ERD-029( )

1 edition published in 2004 in English and held by 0 WorldCat member libraries worldwide

The {delta} to {alpha}' phase transformation in Pu-Ga alloys is intriguing for both scientific and technological reasons. On cooling, the ductile fcc {delta}-phase transforms martensitically to the brittle monoclinic {alpha}'-phase at approximately -120 C (depending on composition). This exothermic transformation involves a 20% volume contraction and a significant increase in resistivity. The reversion of {alpha}' to {delta} involves a large temperature hysteresis beginning just above room temperature. In an attempt to better understand the underlying thermodynamics and kinetics responsible for these unusual features, we have investigated the {delta} {leftrightarrow} {alpha}' phase transformations in a Pu-0.6 wt% Ga alloy using a combination of experimental and modeling techniques
Transformation Crystallography and Plasticity of the Delta to Alpha Prime Transformation in Plutonium Alloys( )

1 edition published in 2003 in English and held by 0 WorldCat member libraries worldwide

In delta phase Pu-Ga alloys, the transformation from the ductile face-centered cubic (fcc) {delta} phase that is retained at room temperature to the brittle low-temperature monoclinic alpha' phase is a thermally activated diffusionless transformation with double-c kinetics. Accurate modeling of the phase transformation requires detailed understanding of the role of plastic flow during the transformation and of the crystallographic transformation path. Using transmission electron microscopy (TEM), we find a significant increase in dislocation density in {delta} near the {alpha}' plates, which suggests that plastic deformation contributes to the accommodation of the 20% reduction in volume during the transformation. Analysis of a series of optical micrographs of partially transformed alloys suggests that the {alpha}' habit plane is usually nearly perpendicular to <111> {delta}. However, a small number of TEM observations support a habit plane near <112> or <123>, in agreement with earlier work
 
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English (13)