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The 300 Kpc Long X-Ray Jet in PKS 1127-145, Z=1.18 Quasar: Constraining X-Ray Emission Models (open access)

The 300 Kpc Long X-Ray Jet in PKS 1127-145, Z=1.18 Quasar: Constraining X-Ray Emission Models

We present a {approx} 100 ksec Chandra X-ray observation and new VLA radio data of the large scale, 300 kpc long X-ray jet in PKS 1127-145, a radio loud quasar at redshift z=1.18. With this deep X-ray observation we now clearly discern the complex X-ray jet morphology and see substructure within the knots. The X-ray and radio jet intensity profiles are seen to be strikingly different with the radio emission peaking strongly at the two outer knots while the X-ray emission is strongest in the inner jet region. The jet X-ray surface brightness gradually decreases by an order of magnitude going out from the core. The new X-ray data contain sufficient counts to do spectral analysis of the key jet features. The X-ray energy index of the inner jet is relatively flat with {alpha}{sub x} = 0.66 {+-} 0.15 and steep in the outer jet with {alpha}{sub x} = 1.0 {+-} 0.2. We discuss the constraints implied by the new data on the X-ray emission models and conclude that ''one-zone'' models fail and at least a two component model is needed to explain the jet's broad-band emission. We propose that the X-ray emission originates in the jet proper while the …
Date: November 20, 2006
Creator: Siemiginowska, Aneta; Stawarz, Lukasz; Cheung, C. C.; Harris, D. E.; Sikora, Marek; Aldcroft, Thomas L. et al.
System: The UNT Digital Library
6th US-Russian Pu Science Workshop Lawrence Livermore National Laboratory University of California, Livermore, California, July 14 and 15, 2006 (open access)

6th US-Russian Pu Science Workshop Lawrence Livermore National Laboratory University of California, Livermore, California, July 14 and 15, 2006

None
Date: June 20, 2006
Creator: Fluss, M.; Tobin, J.; Schwartz, A.; Petrovtsev, A. V.; Nadykto, B. A.; Timofeeva, L. F. et al.
System: The UNT Digital Library
Air to Muscle and Blood/Plasma to Muscle Distribution of Volatile Organic Compounds and Drugs: Linear Free Energy Analyses (open access)

Air to Muscle and Blood/Plasma to Muscle Distribution of Volatile Organic Compounds and Drugs: Linear Free Energy Analyses

Article on air to muscle and blood/plasma to muscle distribution of volatile organic compounds and drugs and linear free energy analyses.
Date: April 20, 2006
Creator: Abraham, M. H. (Michael H.); Ibrahim, Adam & Acree, William E. (William Eugene)
System: The UNT Digital Library
Analytic model of bunched beams for harmonic generation in thelow-gain free electron laser regime (open access)

Analytic model of bunched beams for harmonic generation in thelow-gain free electron laser regime

One scheme for harmonic generation employs free electron lasers (FELs) with two undulators: the first uses a seed laser to modulate the energy of the electron beam; following a dispersive element which acts to bunch the beam, the second undulator radiates at a higher harmonic. These processes are currently evaluated using extensive calculations or simulation codes which can be slow to evaluate and difficult to set up. We describe a simple algorithm to predict the output of a harmonic generation beamline in the low-gain FEL regime, based on trial functions for the output radiation. Full three-dimensional effects are included. This method has been implemented as a Mathematica package, named CAMPANILE, which runs rapidly and can be generalized to include effects such as asymmetric beams and misalignments. This method is compared with simulation results using the FEL code GENESIS, both for single stages of harmonic generation and for the LUX project, a design concept for an ultrafast X-ray facility, where multiple stages upshift the input laser frequency by factors of up to 200.
Date: February 20, 2006
Creator: Penn, G.; Reinsch, M. & Wurtele, J.S.
System: The UNT Digital Library
The BaBar Electromagnetic Calorimeter: Status and Performance Improvements (open access)

The BaBar Electromagnetic Calorimeter: Status and Performance Improvements

The electromagnetic calorimeter at the BABAR detector, part of the asymmetric B Factory at SLAC, measures photons in the energy range from 20 MeV to 8 GeV with high resolution. The current status of the calorimeter, now in its seventh year of operation, is being presented, as well as details on improvements made to the analysis code during the last years.
Date: January 20, 2006
Creator: Bauer, Johannes M.
System: The UNT Digital Library
Beam Delivery WG Summary: Optics, Collimation & Background (open access)

Beam Delivery WG Summary: Optics, Collimation & Background

The presented paper partially summarizes the work of the Beam Delivery working group (WG4) at Snowmass, concentrating on status of optics, layout, collimation, and background. The strawman layout with 2 interaction regions was recommended at the first ILC workshop at KEK in November 2004. Two crossing-angle designs were included in this layout. The design of the ILC BDS has evolved since the first ILC workshop. The progress on the BDS design and extraction line design has been reviewed and the design issues were discussed during the optics and layout session at the Snowmass.
Date: January 20, 2006
Creator: Angal-Kalinin, D.; Jackson, F.; Mokhov, N. V.; Kuroda, S. & Seryi, A. A.
System: The UNT Digital Library
Behavior of Explosives Under Pressure in a Diamond Anvil Cell (open access)

Behavior of Explosives Under Pressure in a Diamond Anvil Cell

Diamond anvil cell (DAC) studies can yield information about the pressure dependence of materials and reactions under conditions comparable to shock loading. The pressure gradient across the face of the diamonds is often deliberately minimized to create uniform pressure over much of the sample and a simplified data set. To reach very high pressures (30-40 GPa), however, it may be necessary to use ''softer'', high nitrogen content diamonds that are more susceptible to bending under pressure. The resulting enhanced pressure gradient then provides a view of high-pressure behavior under anisotropic conditions similar to those found at the burn front in a bulk sample. We discuss visual observations of pressure-induced changes relative to variations in burn rate of several explosives (Triaminotrinitrobenzene, Nitromethane, CL-20) in the DAC. The burn rate behavior of both Nitromethane (NM) and Triaminotrinitrobenzene (TATB) were previously reported for pressures up to {approx}40 GPa. Nitromethane showed a near monotonic increase in burn rate to a maximum at {approx}30 GPa after which the burn rate decreased, all without color change. At higher pressures, the TATB samples had shiny (metallic) polycrystalline zones or inclusions where the pressure was highest in the sample. Around the shiny zones was a gradation of color …
Date: June 20, 2006
Creator: Foltz, M F
System: The UNT Digital Library
Calculation of Radiative Corrections to Hyperfine Splitting in p1/2 States (open access)

Calculation of Radiative Corrections to Hyperfine Splitting in p1/2 States

Techniques to calculate one-loop radiative corrections to hyperfine splitting including binding corrections to all orders have been developed in the last decade for s states of atoms and ions. In this paper these methods are extended to p{sub 1/2} states for three cases. In the first case, the point-Coulomb 2p{sub 1/2} hyperfine splitting is treated for the hydrogen isoelectronic sequence, and the lowest order result, {alpha}/4{pi} E{sub F}, is shown to have large binding corrections at high Z. In the second case, neutral alkalis are considered. In the third case, hyperfine splitting of the 2p{sub 1/2} state of lithium-like bismuth is treated. In the latter two cases, correlation corrections are included and, in addition, the point is stressed that uncertainties associated with nuclear structure, which complicate comparison with experiment for s states, are considerably reduced because of the smaller overlap with the nucleus.
Date: September 20, 2006
Creator: Sapirstein, J & Cheng, K T
System: The UNT Digital Library
Cham and Charmoniium Spectroscopy (open access)

Cham and Charmoniium Spectroscopy

The last few years have seen a revival of interest in charm spectroscopy with more than a dozen new states being reported and hundreds of new theoretical investigations being published. The advent of the B-factories [1,2], with their large, charm-rich data samples, has proven crucial to the discovery and investigation of new charm hadron states, but other experiments have confirmed and complemented the B-factory observations. Much interest has been generated by several new states that do not appear to be easily incorporated in the conventional picture of charm and charmonium mesons. Here, the focus is on the latest experimental results in charm spectroscopy and the determination of the nature of the recently discovered states. Recent experimental results in charm and charmonium spectroscopy are reviewed.
Date: October 20, 2006
Creator: Petersen, Brian Aa.
System: The UNT Digital Library
Comment on "Steady State Solutions to PBPK Models and their Applications to Risk Assessment I: Route to Route Extrapolation of Volatile Chemicals," by Chiu and White in Risk Analysis, 26(3), 769-780 (open access)

Comment on "Steady State Solutions to PBPK Models and their Applications to Risk Assessment I: Route to Route Extrapolation of Volatile Chemicals," by Chiu and White in Risk Analysis, 26(3), 769-780

Steady-state analyses of generic PBPK models for volatile organic chemical (VOC) exposure and risk assessment have been undertaken and applied for nearly two decades now. Chiu and White's paper on this subject adds little new to this earlier work. Their dismissive claim that ''Similar analyses have been done for specific chemicals and for inhalation'' is misleading, because some of this earlier work did indeed focus on ''generic'' PBPK models generally applicable to VOC exposure by multiple routes. In particular, the earliest of these previous studies developed steady-state solutions for generic PBPK models including respiratory and 1-compartment oral routes of exposure, and further specified how to add injection and dermal exposure routes. Chiu and White included a 2-compartment oral pathway and a lung compartment in an otherwise identical generic PBPK model, but did not consider other exposure pathways such as dermal uptake. Each of the earlier studies first presented a steady-state solution to a generic, multiroute PBPK model, and only then applied the generic solution to a problem or illustration involving a specific compound--i.e., the same approach used later by Chiu and White. For example, the earlier study included a simple, intuitive expression for low-dose metabolized fraction f*{sub m} of any …
Date: July 20, 2006
Creator: Bogen, K T
System: The UNT Digital Library
The complete chloroplast genome sequence of Pelargonium xhortorum: Or ganization and evolution of the largest and most highlyrearranged chloroplast genome of land plants (open access)

The complete chloroplast genome sequence of Pelargonium xhortorum: Or ganization and evolution of the largest and most highlyrearranged chloroplast genome of land plants

The chloroplast genome of Pelargonium e hortorum has beencompletely sequenced. It maps as a circular molecule of 217,942 bp, andis both the largest and most rearranged land plant chloroplast genome yetsequenced. It features two copies of a greatly expanded inverted repeat(IR) of 75,741 bp each, and consequently diminished single copy regionsof 59,710 bp and 6,750 bp. It also contains two different associations ofrepeated elements that contribute about 10 percent to the overall sizeand account for the majority of repeats found in the genome. Theyrepresent hotspots for rearrangements and gene duplications and include alarge number of pseudogenes. We propose simple models that account forthe major rearrangements with a minimum of eight IR boundary changes and12 inversions in addition to a several insertions of duplicated sequence.The major processes at work (duplication, IR expansion, and inversion)have disrupted at least one and possibly two or three transcriptionaloperons, and the genes involved in these disruptions form the core of thetwo major repeat associations. Despite the vast increase in size andcomplexity of the genome, the gene content is similar to that of otherangiosperms, with the exceptions of a large number of pseudogenes as partof the repeat associations, the recognition of two open reading frames(ORF56 and ORF42) …
Date: January 20, 2006
Creator: Chumley, Timothy W.; Palmer, Jeffrey D.; Mower, Jeffrey P.; Fourcade, H. Matthew; Calie, Patrick J.; Boore, Jeffrey L. et al.
System: The UNT Digital Library
Conductivity of carbon nanotube polymer composites (open access)

Conductivity of carbon nanotube polymer composites

Dissipative Particle Dynamics (DPD) simulations were used to investigate methods of controlling the assembly of percolating networks of carbon nanotubes (CNTs) in thin films of block copolymer melts. For suitably chosen polymers the CNTs were found to spontaneously self-assemble into topologically interesting patterns. The mesoscale morphology was projected onto a finite-element grid and the electrical conductivity of the films computed. The conductivity displayed non-monotonic behavior as a function of relative polymer fractions in the melt. Results are compared and contrasted with CNT dispersion in small-molecule fluids and mixtures.
Date: November 20, 2006
Creator: Wescott, J. T.; Kung, P. & Maiti, A.
System: The UNT Digital Library
Corrosion Resistances of Iron-Based Amorphous Metals with Yttrium and Tungsten Additions in Hot Calcium Chloride Brine & Natural Seawater: Fe48Mo14Cr15Y2C15B6 and W-Containing Variants (open access)

Corrosion Resistances of Iron-Based Amorphous Metals with Yttrium and Tungsten Additions in Hot Calcium Chloride Brine & Natural Seawater: Fe48Mo14Cr15Y2C15B6 and W-Containing Variants

Yttrium-containing SAM1651 (Fe{sub 48.0}Cr{sub 15.0}Mo{sub 14.0}B{sub 6.0}C{sub 15.0}Y{sub 2.0}), has a critical cooling rate (CCR) of approximately 80 Kelvin per second, while SAM2X5 (Fe{sub 49.7}Cr{sub 17.7}Mn{sub 1.9}Mo{sub 7.4}W{sub 1.6}B{sub 15.2}C{sub 3.8}Si{sub 2.4}) with no yttrium has a higher critical cooling rate of approximately 600 Kelvin per second. SAM1651's low CCR enables it to be rendered as a completely amorphous material in practical materials processes. Chromium (Cr), molybdenum (Mo) and tungsten (W) provide corrosion resistance; boron (B) enables glass formation; and rare earths such as yttrium (Y) lower critical cooling rate (CCR). The passive film stability of these Fe-based amorphous metal formulations have been found to be superior to that of conventional stainless steels, and comparable to that of Ni-based alloys, based on electrochemical measurements of the passive film breakdown potential and general corrosion rates.
Date: October 20, 2006
Creator: Farmer, J C; Haslam, J; Day, S; Lian, T; Saw, C; Hailey, P et al.
System: The UNT Digital Library
Coupling Magnetic Fields and ALE Hydrodynamics for 3D Simulations of MFCG's (open access)

Coupling Magnetic Fields and ALE Hydrodynamics for 3D Simulations of MFCG's

We review the development of a full 3D multiphysics code for the simulation of explosively driven Magnetic Flux Compression Generators (MFCG) and related pulse power devices. In a typical MFCG the device is seeded with an initial electric current and the device is then detonated. The detonation compresses the magnetic field and amplifies the current. This is a multiphysics problem in that detonation kinetics, electromagnetic diffusion and induction, material deformation, and thermal effects are all important. This is a tightly coupled problem in that the different physical quantities have comparable spatial and temporal variation, and hence should be solved simultaneously on the same computational mesh.
Date: September 20, 2006
Creator: White, D; Rieben, R & Wallin, B
System: The UNT Digital Library
DAPHNE Operation And Plans for DAPHNE2 (open access)

DAPHNE Operation And Plans for DAPHNE2

The e{sup +}e{sup -} collider DA{Phi}NE, a 1.02 GeV c.m. {Phi}-factory, has reached a peak luminosity of about 1.4 x 10{sup 32} cm{sup -2} s{sup -1} and a peak integrated luminosity in one day of about 8.6 pb{sup -1}. With the current rates the physics program of the three main experiments DEAR, FINUDA and KLOE will be completed by the end of 2007. In this paper we describe in detail the steps which have led to the luminosity improvement and the options for the upgrade of the collider towards higher energy and/or luminosity.
Date: January 20, 2006
Creator: Zobov, M.; Alesini, D.; Benedetti, G.; Biagini, M. E.; Biscari, C.; Boni, R. et al.
System: The UNT Digital Library
Defect reduction in (112_O) a-plane GaN by two-stage epitaxiallateral overgrowth (open access)

Defect reduction in (112_O) a-plane GaN by two-stage epitaxiallateral overgrowth

In the epitaxial lateral overgrowth (ELO) of (11{bar 2}0) a-plane GaN, the uneven growth rates of two opposing wings, Ga- and N-wings, makes the coalescence of two neighboring wings more difficult than that in c-plane GaN. We report a two-stage growth method to get uniformly coalesced epitaxial lateral overgrown a-plane GaN using metalorganic chemical vapor deposition (MOCVD) by employing relatively lower growth temperature in the first step followed by enhanced lateral growth in the second. Using this method, the height differences between Ga-polar and N-polar wings at the coalescence front could be reduced, thereby making the coalescence of two wings much easier. Transmission electron microscopy (TEM) showed that the threading dislocation density in the wing areas was 1.0x10{sup 8}cm{sup -2}, more than two orders of magnitude lower than that in the window areas (4.2x10{sup 10}cm{sup -2}). However, high density of basal stacking faults of 1.2x104 cm-1 was still observed in the wing areas as compared to c-plane GaN. Atomic force microscopy and photoluminescence measurements on the coalesced ELO a-GaN sample also indicated improved material quality.
Date: October 20, 2006
Creator: Ni, X.; Ozgur, U.; Fu, Y.; Biyikli, N.; Xie, J.; Baski, A.A. et al.
System: The UNT Digital Library
Defining an end state for CO2 sequestration and EOR in North America (open access)

Defining an end state for CO2 sequestration and EOR in North America

CO{sub 2} capture and storage (CCS) presents a challenge to long-range planners, economic interests, regulators, law-makers, and other stakeholders and decision makers. To improve and optimize the use of limited resources and finances, it is important to define an end state for CCS. This ends state should be defined around desired goals and reasonable timelines for execution. While this definition may have substantial technology, policy or economic implications, it need not be prescriptive in terms of technology pathway, policy mechanism, or economic targets. To illustrate these concerns, this paper will present a credible vision of what an end state for North American might look like. From that, examples of key investment and planning decisions are provided to illustrate the value of end-state characterization.
Date: April 20, 2006
Creator: Friedmann, S J
System: The UNT Digital Library
DEFLAGRATION-TO-DETONATION TRANSITION IN LX-04 AS A FUNCTION OF LOADING DENSITY, TEMPERATURE, AND CONFINEMENT (open access)

DEFLAGRATION-TO-DETONATION TRANSITION IN LX-04 AS A FUNCTION OF LOADING DENSITY, TEMPERATURE, AND CONFINEMENT

The potential for deflagration-to-detonation transition (DDT) in LX-04 (85/15 HMX/Viton) is being evaluated as a function of loading density, temperature, and confinement. In the high confinement arrangement, a matrix of tests is nearly completed with the LX-04 loaded at {approx} 51, 70, 90, and {approx} 99% of theoretical maximum density (TMD); and temperatures of ambient, 160 C, and 190 C at each loading density. A more limited set of tests with {approx}99 %TMD loadings at medium confinement were conducted at temperatures of ambient and 186 C. LX-04 does not undergo DDT at near TMD loadings in both medium and high confinement, although the latter still results in significant fragmentation. Most porous beds in high confinement undergo DDT, with the minimum run distance to detonation (l) for a 70 %TMD loading at ambient temperature. LX-04 does not transit to detonation for a pour density (51.3 %TMD) loading at 160 C, but does at 190 C with a longer l than at ambient. The limited ambient temperature measurements for l in high confinement are similar to previous data for 91/9 HMX/wax, which has nearly the same %volume of HMX as LX-04.
Date: June 20, 2006
Creator: Sandusky, Harold W.; Granholm, Richard H.; Bohl, Douglas G.; Vandersall, Kevin S.; Hare, David E. & Garcia, Frank
System: The UNT Digital Library
DESIGN OF A MICROFABRICATED, TWO-ELECTRODE PHASE-CONTRAST ELEMENTSUITABLE FOR ELECTRON MICROSCOPY (open access)

DESIGN OF A MICROFABRICATED, TWO-ELECTRODE PHASE-CONTRAST ELEMENTSUITABLE FOR ELECTRON MICROSCOPY

A miniature electrostatic element has been designed to selectively apply a ninety-degree phase shift to the unscattered beam in the back focal plane of the objective lens, in order to realize Zernike-type, in-focus phase contrast in an electron microscope. The design involves a cylindrically shaped, biased-voltage electrode, which is surrounded by a concentric grounded electrode. Electrostatic calculations have been used to determine that the fringing fields in the region of the scattered electron beams will cause a negligible phase shift as long as the ratio of electrode length to the transverse feature-size is greater than 5:1. Unlike the planar, three-electrode einzel lens originally proposed by Boersch for the same purpose, this new design does not require insulating layers to separate the biased and grounded electrodes, and it can thus be produced by a very simple microfabrication process. Scanning electron microscope images confirm that mechanically robust devices with feature sizes of {approx}1 {micro}m can be easily fabricated. Preliminary experimental images demonstrate that these devices do apply a 90-degree phase shift between the scattered and unscattered electrons, as expected.
Date: September 20, 2006
Creator: Cambie, Rossana; Downing, Kenneth H.; Typke, Dieter; Glaeser,Robert M. & Jin, Jian
System: The UNT Digital Library
The Design Parameters for the MICE Tracker Solenoid (open access)

The Design Parameters for the MICE Tracker Solenoid

The first superconducting magnets to be installed in the muon ionization cooling experiment (MICE) will be the tracker solenoids. The tracker solenoid module is a five coil superconducting solenoid with a 400 mm diameter warm bore that is used to provide a 4 T magnetic field for the experiment tracker module. Three of the coils are used to produce a uniform field (up to 4 T with better than 1 percent uniformity) in a region that is 300 mm in diameter and 1000 mm long. The other two coils are used to match the muon beam into the MICE cooling channel. Two 2.94-meter long superconducting tracker solenoid modules have been ordered for MICE. The tracker solenoid will be cooled using two-coolers that produce 1.5 W each at 4.2 K. The magnet system is described. The decisions that drive the magnet design will be discussed in this report.
Date: August 20, 2006
Creator: Green, Michael A.; Chen, C. Y.; Juang, Tiki; Lau, Wing W.; Taylor, Clyde; Virostek, Steve P. et al.
System: The UNT Digital Library
Development of a Fan-Filter Unit Test Standard, Laboratory Validations, and its Applications across Industries (open access)

Development of a Fan-Filter Unit Test Standard, Laboratory Validations, and its Applications across Industries

Lawrence Berkeley National Laboratory (LBNL) is now finalizing the Phase 2 Research and Demonstration Project on characterizing 2-foot x 4-foot (61-cm x 122-cm) fan-filter units in the market using the first-ever standard laboratory test method developed at LBNL.[1][2][3] Fan-filter units deliver re-circulated air and provide particle filtration control for clean environments. Much of the energy in cleanrooms (and minienvironments) is consumed by 2-foot x 4-foot (61-cm x 122-cm) or 4-foot x 4-foot (122-cm x 122-cm) fan-filter units that are typically located in the ceiling (25-100% coverage) of cleanroom controlled environments. Thanks to funding support by the California Energy Commission's Industrial Program of the Public Interest Energy Research (PIER) Program, and significant participation from manufacturers and users of fan-filter units from around the world, LBNL has developed and performed a series of standard laboratory tests and reporting on a variety of 2-foot x 4-foot (61-cm x 122-cm) fan-filter units (FFUs). Standard laboratory testing reports have been completed and reported back to anonymous individual participants in this project. To date, such reports on standard testing of FFU performance have provided rigorous and useful data for suppliers and end users to better understand, and more importantly, to quantitatively characterize performance of FFU products …
Date: October 20, 2006
Creator: Xu, Tengfang
System: The UNT Digital Library
A Directed Energy System for Defeat of Improvised Explosive Devices and Landmines (open access)

A Directed Energy System for Defeat of Improvised Explosive Devices and Landmines

We describe a laser system, built in our laboratory at LLNL, that has near-term, effective applications in exposing and neutralizing improvised explosive devices and landmines. We discuss experiments with this laser, demonstrating excavation capabilities and relevant material interactions. Model results are also described.
Date: March 20, 2006
Creator: Boley, C.; Fochs, S.; Parker, J.; Rotter, M.; Rubenchik, A. & Yamamoto, R.
System: The UNT Digital Library
Discovering a Defect that Imposes a Limit to Mg Doping in p-TypeGaN (open access)

Discovering a Defect that Imposes a Limit to Mg Doping in p-TypeGaN

Gallium nitride (GaN) is the III-V semiconductor used to produce blue light-emitting diodes (LEDs) and blue and ultraviolet solid-state lasers. To be useful in electronic devices, GaN must be doped with elements that function either as electron donors or as acceptors to turn it into either an n-type semiconductor or a p-type semiconductor. It has been found that GaN can easily be grown with n-conductivity, even up to large concentrations of donors--in the few 10{sup 19}cm{sup -3} range. However, p-doping, the doping of the structure with atoms that provide electron sinks or holes, is not well understood and remains extremely difficult. The only efficient p-type dopant is Mg, but it is found that the free hole concentration is limited to 2 x 10{sup 18}cm{sup -3}, even when Mg concentrations are pushed into the low 10{sup 19}cm{sup -3} range. This saturation effect could place a limit on further development of GaN based devices. Further increase of the Mg concentration, up to 1 x 10{sup 20}cm{sup -3} leads to a decrease of the free hole concentration and an increase in defects. While low- to medium-brightness GaN light-emitting diodes (LEDs) are remarkably tolerant of crystal defects, blue and UV GaN lasers are much …
Date: July 20, 2006
Creator: Liliental-Weber, Z.; Tomaszewicz, T.; Zakharov, D. & O'Keefe, M. A.
System: The UNT Digital Library
Dislocation Multi-junctions and Strain Hardening (open access)

Dislocation Multi-junctions and Strain Hardening

At the microscopic scale, the strength of a crystal derives from the motion, multiplication and interaction of distinctive line defects--dislocations. First theorized in 1934 to explain low magnitudes of crystal strength observed experimentally, the existence of dislocations was confirmed only two decades later. Much of the research in dislocation physics has since focused on dislocation interactions and their role in strain hardening: a common phenomenon in which continued deformation increases a crystal's strength. The existing theory relates strain hardening to pair-wise dislocation reactions in which two intersecting dislocations form junctions tying dislocations together. Here we report that interactions among three dislocations result in the formation of unusual elements of dislocation network topology, termed hereafter multi-junctions. The existence of multi-junctions is first predicted by Dislocation Dynamics (DD) and atomistic simulations and then confirmed by the transmission electron microscopy (TEM) experiments in single crystal molybdenum. In large-scale Dislocation Dynamics simulations, multi-junctions present very strong, nearly indestructible, obstacles to dislocation motion and furnish new sources for dislocation multiplication thereby playing an essential role in the evolution of dislocation microstructure and strength of deforming crystals. Simulation analyses conclude that multi-junctions are responsible for the strong orientation dependence of strain hardening in BCC crystals.
Date: June 20, 2006
Creator: Bulatov, V.; Hsiung, L.; Tang, M.; Arsenlis, A.; Bartelt, M.; Cai, W. et al.
System: The UNT Digital Library