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Articles 1 - 30 of 312

Full-Text Articles in Engineering

Spectroscopic Ellipsometry Study Of Optical Anisotropy In Gd5si2ge2 And Comparison With Reflectance Difference Spectra, Joong Mok Park, David W. Lynch, S. J. Lee, Deborah L. Schlagel, Thomas A. Lograsso, A. O. Tsokol, J. E. Snyder, David C. Jiles Jun 2016

Spectroscopic Ellipsometry Study Of Optical Anisotropy In Gd5si2ge2 And Comparison With Reflectance Difference Spectra, Joong Mok Park, David W. Lynch, S. J. Lee, Deborah L. Schlagel, Thomas A. Lograsso, A. O. Tsokol, J. E. Snyder, David C. Jiles

David C. Jiles

The complex dielectric functions of single crystals of Gd5Si2Ge2 were obtained using spectroscopic ellipsometry (SE) in the photon energy range of 1.5–5.0 eV at room temperature. Reflectance difference (RD) spectra for the a‐b and b‐c planes of single crystals of Gd5Si2Ge2 were derived from these dielectric functions and compared to those obtained from reflectance difference spectroscopy (RDS) at near-normal incidence. The two experimental RD spectra from SE and RDS agreed well. The in-plane optical anisotropy of the sample is mainly due to intrinsic bulk properties because of its larger magnitude (4×10−2) compared to ...


Metals Additive Manufacturing Powder Aging Characterization, Thomas Russell Lovejoy, Nicholas Karl Muetterties, David Takeo Otsu Jun 2016

Metals Additive Manufacturing Powder Aging Characterization, Thomas Russell Lovejoy, Nicholas Karl Muetterties, David Takeo Otsu

Mechanical Engineering

The metallic additive manufacturing process known as selective laser melting requires highly spherical, normally distributed powder with diameters in the range of 10 to 50 microns. Previous observations have shown a degradation in powder quality over time, resulting in unwanted characteristics in the final printed parts. 21-6-9 stainless steel powder was used to fabricate test parts, with leftover powder recycled back into the machine. Powder samples and test specimens were characterized to observe changes across build cycles. Few changes were observed in the physical and mechanical properties of the specimens, however, there were indications of chemical changes across cycles. Potential ...


Second Order Phase Transition Temperature Of Single Crystals Of Gd5si1.3ge2.7 And Gd5si1.4ge2.6, Ravi L. Hadimani, Yevgen Melikhov, Deborah L. Schlagel, Thomas A. Lograsso, Kevin W. Dennis, R. William Mccallum, David C. Jiles May 2016

Second Order Phase Transition Temperature Of Single Crystals Of Gd5si1.3ge2.7 And Gd5si1.4ge2.6, Ravi L. Hadimani, Yevgen Melikhov, Deborah L. Schlagel, Thomas A. Lograsso, Kevin W. Dennis, R. William Mccallum, David C. Jiles

David C. Jiles

Gd5(Six Ge 1−x)4 has mixed phases in the composition range 0.32 < x < 0.41, which have not been widely studied. In this paper, we have synthesized and indexed single crystal samples of Gd5Si1.3 Ge 2.7 and Gd5Si1.4 Ge 2.6. We have investigated the first order and second orderphase transition temperatures of these samples using magnetic moment vs. temperature andmagnetic moment vs. magnetic field at different temperatures. We have used a modified Arrott plot technique that was developed and reported by us previously to determine the “hidden” second order phase transition temperature of ...


Corrosion Behavior Of Steel In Deficient Grout With Enhanced Sulfate Ion Concentrations, Krishna Vigneshwaran Konda Krishnamurthy Apr 2016

Corrosion Behavior Of Steel In Deficient Grout With Enhanced Sulfate Ion Concentrations, Krishna Vigneshwaran Konda Krishnamurthy

FIU Electronic Theses and Dissertations

Recent corrosion failures of tendons in Florida post-tensioned (PT) bridges utilizing low bleed specified grout products have shown the need for better understanding of the mechanisms that cause corrosion and to determine the extent of the problem in PT tendons with similar materials. Those deficient grouts in the tendons showed low chloride content below commonly assumed threshold limits for corrosion initiation, but they showed higher concentrations of sulfate ions, high pore water pH and higher moisture content. The role of enhanced sulfate ion concentrations in the development of steel corrosion in deficient grout has not been elucidated, but the role ...


Surface Structures Determiend By Kinetic Processes: Adsorption And Diffusion Of Oxygen On Pd(100), S. -L. Chang, D. E. Sanders, James W. Evans, Patricia A. Thiel Feb 2016

Surface Structures Determiend By Kinetic Processes: Adsorption And Diffusion Of Oxygen On Pd(100), S. -L. Chang, D. E. Sanders, James W. Evans, Patricia A. Thiel

James W. Evans

Atomic oxygen forms a metastable c(2×2) phase on Pd(100) under conditions of rapid adsorption (high pressure) and slow diffusion (low sample temperature). One possible explanation is that oxygen molecules require an 8-fold ensemble of empty sites for dissociative chemisorption, and that subsequent adatom motion is limited and creates no neighboring pairs of filled sites. We describe the properties of the adlayer predicted by such a model.


Structure And Growth Of Height-Selected Ag Islands On Fivefold I-Alpdmn Quasicrystalline Surfaces: Stm Analysis And Step Dynamics Modeling, Barış Ünal, V. Fournée, Patricia A. Thiel, James W. Evans Feb 2016

Structure And Growth Of Height-Selected Ag Islands On Fivefold I-Alpdmn Quasicrystalline Surfaces: Stm Analysis And Step Dynamics Modeling, Barış Ünal, V. Fournée, Patricia A. Thiel, James W. Evans

James W. Evans

The development and local structure of height-selected 3-layer Ag islands on fivefold surfaces of icosahedral Al-Pd-Mn quasicrystals is characterized by STM for Ag deposition at 365 K. Heterogeneous nucleation of pseudomorphic single layer high islands is followed by rapid formation of 2nd and 3rd layers and subsequent lateral spreading, where each of these 3 layers consists of a family of nonfcc structures. The behavior is elucidated by step dynamics modeling incorporating strain buildup for larger islands, enhanced binding in higher layers, and height selection due to quantum size effects.


Scanning Tunneling Microscopy And Density Functional Theory Study Of Initial Bilayer Growth Of Ag Films On Nial(110), Barış Ünal, Feili Qin, Yong Han, Da-Jiang Liu, Dapeng Jing, A. R. Layson, Cynthia J. Jenks, James W. Evans, Patricia A. Thiel Feb 2016

Scanning Tunneling Microscopy And Density Functional Theory Study Of Initial Bilayer Growth Of Ag Films On Nial(110), Barış Ünal, Feili Qin, Yong Han, Da-Jiang Liu, Dapeng Jing, A. R. Layson, Cynthia J. Jenks, James W. Evans, Patricia A. Thiel

James W. Evans

Scanning tunneling microscopy (STM) studies of the deposition of Ag on bcc NiAl(110) in the temperature range from 200 to 300 K reveal an initial bilayer growth mode. In this regime, which encompasses at least the first two levels of bilayer islands, the film appears to have an fcc Ag(110)-like structure. Selection of this structure reflects an almost perfect lateral match between the Ag(110) and NiAl(110) lattice constants. Density functional theory (DFT) analysis of supported Ag films with an ideal fcc(110) structure on NiAl(110) indicates that the bilayer growth mode is promoted by ...


Reconstruction Of Steps On The Cu(111) Surface Induced By Sulfur, Holly L. Walen, Da-Jiang Liu, Junepyo Oh, Hyunseob Lim, James W. Evans, Yousoo Kim, Patricia A. Thiel Feb 2016

Reconstruction Of Steps On The Cu(111) Surface Induced By Sulfur, Holly L. Walen, Da-Jiang Liu, Junepyo Oh, Hyunseob Lim, James W. Evans, Yousoo Kim, Patricia A. Thiel

James W. Evans

Arich menagerie of structures is identified at 5Kfollowing adsorption of lowcoverages (≤0.05 monolayers) of S on Cu(111) at room temperature. This paper emphasizes the reconstructions at the steps. The A-type close-packed step has 1 row of S atoms along its lower edge, where S atoms occupy alternating pseudo-fourfold-hollow (p4fh) sites. Additionally, there are 2 rows of S atoms of equal density on the upper edge, bridging a row of extra Cu atoms, together creating an extended chain. The B-type close-packed step exhibits an even more complex reconstruction, in which triangle-shaped groups of Cu atoms shift out of their ...


Variation Of Growth Morphology With Chemical Composition Of Terraces: Ag On A Twofold Surface Of A Decagonal Al-Cu-Co Quasicrystal, T. Duguet, Barış Ünal, Yong Han, James W. Evans, J. Ledieu, Cynthia J. Jenks, J.-M. Dubois, V. Fournée, Patricia A. Thiel Feb 2016

Variation Of Growth Morphology With Chemical Composition Of Terraces: Ag On A Twofold Surface Of A Decagonal Al-Cu-Co Quasicrystal, T. Duguet, Barış Ünal, Yong Han, James W. Evans, J. Ledieu, Cynthia J. Jenks, J.-M. Dubois, V. Fournée, Patricia A. Thiel

James W. Evans

Growth of Ag thin films on the twofold surface of a decagonal Al-Cu-Co quasicrystal is characterized by scanning tunneling microscopy, at different temperatures, and for coverages ranging from submonolayer to 11 monolayers. From prior work, three types of clean surface terraces are known to exist. By correlation with a bulk structural model, the major difference between them lies in their transition-metal (TM) content, two being aluminum-rich (0 and 15 at. % TM) and one being TM-rich (40–50 at. % TM). The present article focuses on understanding the difference between Ag film morphologies on these terminations, in terms of their chemical content ...


Nucleation And Growth Of Ag Islands On Fivefold Al-Pd-Mn Quasicrystal Surfaces: Dependence Of Island Density On Temperature And Flux, Barış Ünal, V. Fournée, Kyle Schnitzenbaumer, C. Ghosh, Cynthia J. Jenks, A. R. Ross, Thomas A. Lograsso, James W. Evans, Patricia A. Thiel Feb 2016

Nucleation And Growth Of Ag Islands On Fivefold Al-Pd-Mn Quasicrystal Surfaces: Dependence Of Island Density On Temperature And Flux, Barış Ünal, V. Fournée, Kyle Schnitzenbaumer, C. Ghosh, Cynthia J. Jenks, A. R. Ross, Thomas A. Lograsso, James W. Evans, Patricia A. Thiel

James W. Evans

Scanning tunneling microscopy (STM) has been used to investigate the nucleation and growth of Ag islands on the fivefold surface of an icosahedral Al-Pd-Mn quasicrystal. Analysis of the data as a function of deposition temperature, from 127 K to 300 K, reveals that island density is constant, while at higher temperature it decreases. To model this behavior, we first show that the potential energy surface describing bonding of Ag at various locations on the surface is complex, with a few sites acting as traps for clusters of adatoms. We then develop a rate equation model which incorporates enhanced nucleation at ...


Kinetics Of Facile Bilayer Island Formation At Low Temperature: Ag/Nial(110), Yong Han, Barış Ünal, Feili Qin, Dapeng Jing, Cynthia J. Jenks, Da-Jiang Liu, Patricia A. Thiel, James W. Evans Feb 2016

Kinetics Of Facile Bilayer Island Formation At Low Temperature: Ag/Nial(110), Yong Han, Barış Ünal, Feili Qin, Dapeng Jing, Cynthia J. Jenks, Da-Jiang Liu, Patricia A. Thiel, James W. Evans

James W. Evans

Facile nucleation and growth of bilayer Ag(110) islands on NiAl(110) is observed by STM for Ag deposition at temperatures as low as 127 K. Density functional theory analysis for supported Ag films determines adatom adsorption energies (which favor bilayer islands), interaction energies, and diffusion barriers. Analysis of an atomistic lattice-gas model incorporating these energies elucidates the role of strongly anisotropic interactions in enabling the upward mass transport needed for bilayer island formation.


From Initial To Late Stages Of Epitaxial Thin Film Growth: Stm Analysis And Atomistic Or Coarsegrained Modeling, James W. Evans, Yong Han, Barış Ünal, Maozhi Li, K. J. Caspersen, Dapeng Jing, Anthony R. Layson, C. R. Stoldt, T. Duguet, Patricia A. Thiel Feb 2016

From Initial To Late Stages Of Epitaxial Thin Film Growth: Stm Analysis And Atomistic Or Coarsegrained Modeling, James W. Evans, Yong Han, Barış Ünal, Maozhi Li, K. J. Caspersen, Dapeng Jing, Anthony R. Layson, C. R. Stoldt, T. Duguet, Patricia A. Thiel

James W. Evans

Epitaxial thin film growth by vapor deposition or molecular beam epitaxy under ultra‐high vacuum conditions generally occurs in two stages: (i) nucleation and growth of well‐separated islands on the substrate; (ii) subsequent formation of a thicker continuous film with possible kinetic roughening. For homoepitaxial growth, two‐dimensional (2D) monolayer islands are formed during submonolayer deposition. Typically, the presence of a step‐edge barrier inhibits downward transport and leads to the formation of mounds (multilayer stacks of 2D islands) during multilayer growth. For heteroepitaxial growth, islands formed in the initial stages of deposition sometimes have a 2D monolayer structure ...


Formation And Coarsening Of Ag(110) Bilayer Islands On Nial(110): Stm Analysis And Atomistic Lattice-Gas Modeling, Yong Han, Barış Ünal, Dapeng Jing, Feili Qin, Cynthia J. Jenks, Da-Jiang Liu, Patricia A. Thiel, James W. Evans Feb 2016

Formation And Coarsening Of Ag(110) Bilayer Islands On Nial(110): Stm Analysis And Atomistic Lattice-Gas Modeling, Yong Han, Barış Ünal, Dapeng Jing, Feili Qin, Cynthia J. Jenks, Da-Jiang Liu, Patricia A. Thiel, James W. Evans

James W. Evans

Scanning tunneling microscopy analysis of the initial stages of film growth during deposition of Ag on NiAl(110) reveals facile formation of bilayer Ag(110) islands at temperatures of 130 K and above. Annealing subsequent to deposition at 130 K induces coarsening of the bilayer island distribution. The thermodynamic driving force for bilayer island formation reflects a lower relative surface energy for films of even layer thicknesses. This feature derives from quantum size effects due to electron confinement in the Ag film. The kinetics of island formation and relaxation is controlled by terrace and edge-diffusion barriers, detachment barriers, interlayer diffusion ...


Exploration Of Complex Multilayer Film Growth Morphologies: Stm Analysis And Predictive Atomistic Modeling For Ag On Ag(111), Maozhi Li, P.-W. Chung, E. Cox, Cynthia J. Jenks, Patricia A. Thiel, James W. Evans Feb 2016

Exploration Of Complex Multilayer Film Growth Morphologies: Stm Analysis And Predictive Atomistic Modeling For Ag On Ag(111), Maozhi Li, P.-W. Chung, E. Cox, Cynthia J. Jenks, Patricia A. Thiel, James W. Evans

James W. Evans

Scanning tunneling microscopy studies are integrated with development of a realistic atomistic model to both characterize and elucidate the complex mounded morphologies formed by deposition of Ag on Ag(111) at 150 and 180 K. Threefold symmetric lateral shapes of islands and mounds are shown to reflect the influence of a nonuniform step edge barrier inhibiting interlayer transport. Modeling of structure at the mound peaks leads to a sensitive estimate of the magnitude of this large barrier.


Determining Whether Metals Nucleate Homogeneously On Graphite: A Case Study With Copper, David Victor Appy, Huaping Lei, Yong Han, Cai-Zhuang Wang, Michael C. Tringides, Dahai Shao, Emma Jane Kwolek, James W. Evans, Patricia A. Thiel Feb 2016

Determining Whether Metals Nucleate Homogeneously On Graphite: A Case Study With Copper, David Victor Appy, Huaping Lei, Yong Han, Cai-Zhuang Wang, Michael C. Tringides, Dahai Shao, Emma Jane Kwolek, James W. Evans, Patricia A. Thiel

James W. Evans

We observe that Cu clusters grow on surface terraces of graphite as a result of physical vapor deposition in ultrahigh vacuum. We show that the observation is incompatible with a variety of models incorporating homogeneous nucleation and calculations of atomic-scale energetics. An alternative explanation, ion-mediated heterogeneous nucleation, is proposed and validated, both with theory and experiment. This serves as a case study in identifying when and whether the simple, common observation of metal clusters on carbon-rich surfaces can be interpreted in terms of homogeneous nucleation. We describe a general approach for making system-specific and laboratory-specific predictions.


Coarsening Of Two-Dimensional Nanoclusters On Metal Surfaces, Patricia A. Thiel, Mingmin Shen, Da-Jiang Liu, James W. Evans Feb 2016

Coarsening Of Two-Dimensional Nanoclusters On Metal Surfaces, Patricia A. Thiel, Mingmin Shen, Da-Jiang Liu, James W. Evans

James W. Evans

We describe experimental observations and theoretical analysis of the coarsening of distributions of two-dimensional nanoclusters, either adatom islands or vacancy pits, on metal surfaces. A detailed analyses is provided for Ag(111) and Ag(100) surfaces, although we also discuss corresponding behavior for Cu(111) and Cu(100) surfaces. The dominant kinetic pathway for coarsening can be either Ostwald ripening (OR), i.e., growth of larger clusters at the expense of smaller ones, or Smoluchowski ripening (SR), i.e., diffusion and coalescence of clusters. First, for pristine additive-free surfaces, we elucidate the factors which control the dominant pathway. OR kinetics ...


Communication: Structure, Formation, And Equilibration Of Ensembles Of Ag-S Complexes On An Ag Surface, Selena M. Russell, Yousoo Kim, Da-Jiang Liu, James W. Evans, Patricia A. Thiel Feb 2016

Communication: Structure, Formation, And Equilibration Of Ensembles Of Ag-S Complexes On An Ag Surface, Selena M. Russell, Yousoo Kim, Da-Jiang Liu, James W. Evans, Patricia A. Thiel

James W. Evans

We have utilized conditions of very low temperature (4.7 K) and very low sulfur coverage to isolate and identify Ag-S complexes that exist on the Ag(111) surface. The experimental conditions are such that the complexes form at temperatures above the temperature of observation. These complexes can be regarded as polymeric chains of varying length, with an Ag4S pyramid at the core of each monomeric unit. Steps may catalyze the formation of the chains and this mechanism may be reflected in the chain length distribution.


Anisotropic Coarsening: One-Dimensional Decay Of Ag Islands On Ag(110), Yong Han, Selena M. Russell, Anthony R. Layson, Holly L. Walen, Chad D. Yuen, Patricia A. Thiel, James W. Evans Feb 2016

Anisotropic Coarsening: One-Dimensional Decay Of Ag Islands On Ag(110), Yong Han, Selena M. Russell, Anthony R. Layson, Holly L. Walen, Chad D. Yuen, Patricia A. Thiel, James W. Evans

James W. Evans

Scanning tunneling microscopy studies show that coarsening of arrays of rectangular single-layer Ag islands on Ag(110) at 220 K and below occurs by one-dimensional (1D) decay of narrower islands, which maintain roughly constant width in the 〈001〉 direction. Adatoms mainly detach from the island ends with 〈001〉 step edges. 1D decay derives from the absence of corner rounding diffusion from 〈001〉 to 〈1̅ 10〉 edges and from inhibited nucleation of new layers on 〈1̅ 10〉 edges. In contrast, rounding from 〈1̅ 10〉 to 〈001〉 edges is active. The island decay rate exhibits an unexpectedly low effective Arrhenius energy due ...


Accelerated Coarsening Of Ag Adatom Islands On Ag(111) Due To Trace Amounts Of S: Mass-Transport Mediated By Ag–S Complexes, Mingmin Shen, Da-Jiang Liu, Cynthia J. Jenks, Patricia A. Thiel, James W. Evans Feb 2016

Accelerated Coarsening Of Ag Adatom Islands On Ag(111) Due To Trace Amounts Of S: Mass-Transport Mediated By Ag–S Complexes, Mingmin Shen, Da-Jiang Liu, Cynthia J. Jenks, Patricia A. Thiel, James W. Evans

James W. Evans

Scanning tunneling microscopy studies reveal that trace amounts of adsorbed S below a critical coverage on the order of 10 mML have little effect on the coarsening and decay of monolayer Ag adatom islands on Ag(111) at 300 K. In contrast, above this critical coverage, decay is greatly accelerated. This critical value appears to be determined by whether all S can be accommodated at step edges. Accelerated coarsening derives from the feature that the excess S (above that incorporated at steps) produces significant populations on the terraces of metal-sulfur complexes, which are stabilized by strong Ag–S bonding. These ...


Adsorbate-Enhanced Transport Of Metals On Metal Surfaces: Oxygen And Sulfur On Coinage Metals, Patricia A. Thiel, Mingmin Shen, Da-Jiang Liu, James W. Evans Feb 2016

Adsorbate-Enhanced Transport Of Metals On Metal Surfaces: Oxygen And Sulfur On Coinage Metals, Patricia A. Thiel, Mingmin Shen, Da-Jiang Liu, James W. Evans

James W. Evans

Coarsening (i.e., ripening) of single-atom-high, metal homoepitaxial islands provides a useful window on the mechanism and kinetics of mass transport at metal surfaces. This article focuses on this type of coarsening on the surfaces of coinage metals (Cu, Ag, Au), both clean and with an adsorbed chalcogen (O, S) present. For the clean surfaces, three aspects are summarized: (1) the balance between the two major mechanisms—Ostwald ripening (the most commonly anticipated mechanism) and Smoluchowski ripening—and how that balance depends on island size; (2) the nature of the mass transport agents, which are metal adatoms in almost all ...


The Effect Of Macrophage Phenotype And Surface Modification Of Liposomes On Internalization, Lilusi Ma Jan 2016

The Effect Of Macrophage Phenotype And Surface Modification Of Liposomes On Internalization, Lilusi Ma

Graduate Theses and Dissertations

The effects of surface modifications on liposomes using a library of arginine derivatives for improved drug delivery were examined. Encapsulating either fluorescein or doxorubicin, both unmodified liposome and modified liposomes were tested for their drug delivery properties and propensity for internalization with macrophages. The modified liposomes were characterized by dynamic light scattering (DLS) and zeta potential. The resulting liposomes were able to encapsulate doxorubicin with a loading efficiency greater than 90% and cumulative releases of less than 15% after 144 h. The internalization of these particles was examined by loading the liposomes with fluorescein or doxorubicin to test internalization through ...


Oxidation Of Zirconium Diboride Based And Molybdenum Based Ultra-High Temperature Ceramics, Gaoyuan Ouyang Jan 2016

Oxidation Of Zirconium Diboride Based And Molybdenum Based Ultra-High Temperature Ceramics, Gaoyuan Ouyang

Graduate Theses and Dissertations

Designing resilient high temperature alloys has been a long-standing engineering challenge. The driver for high temperature material research ranges from the need of developing robust thermal protection systems for hypersonic vehicles to increased operating temperatures (and hence Carnot efficiencies) of gas turbines. Ni based superalloys have been widely used in a variety of high temperature applications, but these alloys are limited by the melting temperature of Ni3Al and a variety of topologically close-packed phases that melt in the 1200 - 1300°C range. Transitioning to higher temperatures call for materials with high thermal stability. Hence, ceramics such as silicides, borides, nitrides ...


Altering Macrophage And Fibroblast Responses With Biomaterials For Drug Delivery, Wound Healing, And Tissue Engineering, Hannah Bygd Jan 2016

Altering Macrophage And Fibroblast Responses With Biomaterials For Drug Delivery, Wound Healing, And Tissue Engineering, Hannah Bygd

Graduate Theses and Dissertations

Even with significant advances in the development of biomaterials for drug delivery, tissue engineering scaffolds, artificial organs, and other medical devices, one obstacle that remains is a limited understanding of material biocompatibility. Ultimately, the success or failure of these biomaterials depends on the extent of the wound healing and foreign body response following implantation. As macrophage phenotype is dynamic throughout the course of these processes, this research targets these cells to engineer improved materials for modern healthcare applications, and to better understand the material parameters that influence biocompatibility. Evidence shows that polymeric systems can influence the function of macrophages, but ...


Fabrication And Characterization Of Novel Polymer-Matrix Nanocomposites And Their Constituents, Rui Ding Jan 2016

Fabrication And Characterization Of Novel Polymer-Matrix Nanocomposites And Their Constituents, Rui Ding

Graduate Theses and Dissertations

Two main issues for the wide application of polymer-matrix nanocomposites need to be addressed: cost-effective processing of high-performance nanomaterials, and fundamental understanding of the nanofiller-polymer interaction related to property changes of nanocomposites.

To fabricate inexpensive and robust carbon nanofibers (CNFs) by the electrospinning technique, an organosolv lignin for replacing polyacrylonitrile (PAN) precursor was investigated in this work. Modification of lignin to its butyl ester alters the electrospinnability and the thermal mobility of the lignin/PAN blend precursor fibers, which further affect the thermostabilization and carbonization processes of CNFs. The micromorphology, carbon structure, and mechanical properties of resultant CNFs were evaluated ...


Mechanical Deforming The Alloy Body Having Elongated, Ribbon-Shaped Phases Aligned Along Axis Of The Body, John D. Verhoeven, Timothy W. Ellis, Alan M. Russell, Lawrence L. Jones Sep 2015

Mechanical Deforming The Alloy Body Having Elongated, Ribbon-Shaped Phases Aligned Along Axis Of The Body, John D. Verhoeven, Timothy W. Ellis, Alan M. Russell, Lawrence L. Jones

Alan M. Russell

A high strength, light weight "in-situ" Ti-Y composite is produced by deformation processing a cast body having Ti and Y phase components distributed therein. The composite comprises elongated, ribbon-shaped Ti and Y phase components aligned along an axis of the deformed body.


Aluminum/Alkaline Earth Metal Composites And Method For Producing, Alan M. Russell, Iver E. Anderson, Hyongjune Kim, Andrew E. Freichs Sep 2015

Aluminum/Alkaline Earth Metal Composites And Method For Producing, Alan M. Russell, Iver E. Anderson, Hyongjune Kim, Andrew E. Freichs

Alan M. Russell

A composite is provided having an electrically conducting Al matrix andelongated filaments comprising Ca and/or Sr and/or Ba disposed in the matrixand extending along a longitudinal axis of the composite. The filaments initially comprise Ca and/or Sr and/or Ba metal or alloy and then may be reacted with the Al matrix to form a strengthening intermetallic compound comprising Al andCa and/or Sr and/or Ba. The composite is useful as a long-distance, high voltage power transmission conductor.


Magneto-Structural Transformations Via A Solid-State Nudged Elastic Band Method: Application To Iron Under Pressure, Nikolai A. Zarkevich, Duane D. Johnson Sep 2015

Magneto-Structural Transformations Via A Solid-State Nudged Elastic Band Method: Application To Iron Under Pressure, Nikolai A. Zarkevich, Duane D. Johnson

Nikolai A. Zarkevich

We extend the solid-state nudged elastic band method to handle a non-conserved order parameter, in particular, magnetization, that couples to volume and leads to many observed effects in magnetic systems. We apply this formalism to the well-studied magneto-volumecollapse during the pressure-induced transformation in iron—from ferromagnetic body-centered cubic (bcc) austenite to hexagonal close-packed (hcp) martensite. We find a bcc-hcp equilibrium coexistence pressure of 8.4 GPa, with the transition-state enthalpy of 156 meV/Fe at this pressure. A discontinuity in magnetization and coherent stress occurs at the transition state, which has a form of a cusp on the potential-energy surface ...


Accurate And Fast Numerical Solution Of Poisson's Equation For Arbitrary, Space-Filling Voronoi Polyhedra: Near-Field Corrections Revisited, Aftab Alam, Brian G. Wilson, Duane D. Johnson Sep 2015

Accurate And Fast Numerical Solution Of Poisson's Equation For Arbitrary, Space-Filling Voronoi Polyhedra: Near-Field Corrections Revisited, Aftab Alam, Brian G. Wilson, Duane D. Johnson

Duane D. Johnson

We present an accurate and rapid solution of Poisson's equation for space-filling, arbitrarily shaped, convex Voronoi polyhedra (VP); the method is O(NVP), where NVP is the number of distinct VP representing the system. In effect, we resolve the long-standing problem of fast but accurate numerical solution of the near-field corrections, contributions to the potential due to near VP—typically those involving multipole-type conditionally convergent sums, or use of fast Fourier transforms. Our method avoids all ill-convergent sums, is simple, accurate, efficient, and works generally, i.e., for periodic solids, molecules, or systems with disorder or imperfections. We demonstrate ...


Mixed Valency And Site-Preference Chemistry For Cerium And Its Compounds: A Predictive Density-Functional Theory Study, Aftab Alam, Duane D. Johnson Sep 2015

Mixed Valency And Site-Preference Chemistry For Cerium And Its Compounds: A Predictive Density-Functional Theory Study, Aftab Alam, Duane D. Johnson

Duane D. Johnson

Cerium and its technologically relevant compounds are examples of anomalous mixed valency, originating from two competing oxidation states—itinerant Ce4+ and localized Ce3+. Under applied stress, anomalous transitions are observed but not well understood. Here we treat mixed valency as an “alloy” problem involving two valences with competing and numerous site-occupancy configurations. We use density-functional theory with Hubbard U (i.e., DFT+U) to evaluate the effective valence and predict properties, including controlling the valence by pseudoternary alloying. For Ce and its compounds, such as (Ce,La)2(Fe,Co)14B permanent magnets, we find a stable mixed-valent α state ...


Origin Of Spin Gapless Semiconductor Behavior In Cofecrga: Theory And Experiment, Lakhan Bainsla, A. I. Mallick, M. Manivel Raja, A. A. Coelho, A. K. Nigam, Duane D. Johnson, Aftab Alam, K. G. Suresh Jul 2015

Origin Of Spin Gapless Semiconductor Behavior In Cofecrga: Theory And Experiment, Lakhan Bainsla, A. I. Mallick, M. Manivel Raja, A. A. Coelho, A. K. Nigam, Duane D. Johnson, Aftab Alam, K. G. Suresh

Ames Laboratory Publications

Despite a plethora of materials suggested for spintronic applications, a new class of materials has emerged, namely spin gapless semiconductors (SGS), which offers potentially more advantageous properties than existing ones. These magnetic semiconductors exhibit a finite band gap for one spin channel and a closed gap for the other. Here, supported by electronic-structure calculations, we report evidence of SGS behavior in equiatomic quaternary CoFeCrGa, having a cubic Heusler (prototype LiMgPdSn) structure but exhibiting chemical disorder (DO3 structure). CoFeCrGa is found to transform from SGS to half-metallic phase under pressure, which is attributed to unique electronic-structure features. The saturation magnetization (MS ...