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Full-Text Articles in Engineering

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 ...


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 ...


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 Mar 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

Deborah L. Schlagel

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 ...


Effects Of Zn Additions To Highly Magnetoelastic Fega Alloys, Thomas A. Lograsso, Nicholas J. Jones, Deborah L. Schlagel, Gabriela Petculescu, Marilyn Wun-Fogle, James B. Restorff, Arthur E. Clark, Kristi B. Hathaway Mar 2016

Effects Of Zn Additions To Highly Magnetoelastic Fega Alloys, Thomas A. Lograsso, Nicholas J. Jones, Deborah L. Schlagel, Gabriela Petculescu, Marilyn Wun-Fogle, James B. Restorff, Arthur E. Clark, Kristi B. Hathaway

Deborah L. Schlagel

Fe1−xMx (M = Ga, Ge, Si, Al, Mo and x ∼ 0.18) alloys offer an extraordinary combination ofmagnetoelasticity and mechanical properties. They are rare-earth-free, can be processed using conventional deformation techniques, have high magnetic permeability, low hysteresis, and low magnetic saturation fields, making them attractive for device applications such as actuators and energy harvesters. Starting with Fe-Ga as a reference and using a rigid-band-filling argument, Zhang et al. predicted that lowering the Fermi level by reducing the total number of electrons could enhance magnetoelasticity. To provide a direct experimental validation for Zhang's hypothesis, elemental additions with lower-than-Ga valence are ...


Rhombohedral Magnetostriction In Dilute Iron (Co) Alloys, Nicholas J. Jones, Gabriela Petculescu, Marilyn Wun-Fogle, James B. Restorff, Arthur E. Clark, Kristi B. Hathaway, Deborah L. Schlagel, Thomas A. Lograsso Mar 2016

Rhombohedral Magnetostriction In Dilute Iron (Co) Alloys, Nicholas J. Jones, Gabriela Petculescu, Marilyn Wun-Fogle, James B. Restorff, Arthur E. Clark, Kristi B. Hathaway, Deborah L. Schlagel, Thomas A. Lograsso

Deborah L. Schlagel

Iron is a well-utilized material in structural and magnetic applications. This does not mean, however, that it is well understood, especially in the field of magnetostriction. In particular, the rhombohedral magnetostriction of iron, λ111 , is anomalous in two respects: it is negative in sign, in disagreement with the prediction of first principles theory, and its magnitude decreases with increasing temperature much too rapidly to be explained by a power law dependence on magnetization. These behaviors could arise from the location of the Fermi level, which leaves a small region of the majority 3d t2g states unfilled, possibly favoring small internal ...


Effects Of Zn Additions To Highly Magnetoelastic Fega Alloys, Thomas A. Lograsso, Nicholas J. Jones, Deborah L. Schlagel, Gabriela Petculescu, Marilyn Wun-Fogle, James B. Restorff, Arthur E. Clark, Kristi B. Hathaway Mar 2016

Effects Of Zn Additions To Highly Magnetoelastic Fega Alloys, Thomas A. Lograsso, Nicholas J. Jones, Deborah L. Schlagel, Gabriela Petculescu, Marilyn Wun-Fogle, James B. Restorff, Arthur E. Clark, Kristi B. Hathaway

Deborah L. Schlagel

Fe1−xMx (M = Ga, Ge, Si, Al, Mo and x ∼ 0.18) alloys offer an extraordinary combination ofmagnetoelasticity and mechanical properties. They are rare-earth-free, can be processed using conventional deformation techniques, have high magnetic permeability, low hysteresis, and low magnetic saturation fields, making them attractive for device applications such as actuators and energy harvesters. Starting with Fe-Ga as a reference and using a rigid-band-filling argument, Zhang et al. predicted that lowering the Fermi level by reducing the total number of electrons could enhance magnetoelasticity. To provide a direct experimental validation for Zhang's hypothesis, elemental additions with lower-than-Ga valence are ...


Rhombohedral Magnetostriction In Dilute Iron (Co) Alloys, Nicholas J. Jones, Gabriela Petculescu, Marilyn Wun-Fogle, James B. Restorff, Arthur E. Clark, Kristi B. Hathaway, Deborah L. Schlagel, Thomas A. Lograsso Mar 2016

Rhombohedral Magnetostriction In Dilute Iron (Co) Alloys, Nicholas J. Jones, Gabriela Petculescu, Marilyn Wun-Fogle, James B. Restorff, Arthur E. Clark, Kristi B. Hathaway, Deborah L. Schlagel, Thomas A. Lograsso

Deborah L. Schlagel

Iron is a well-utilized material in structural and magnetic applications. This does not mean, however, that it is well understood, especially in the field of magnetostriction. In particular, the rhombohedral magnetostriction of iron, λ111 , is anomalous in two respects: it is negative in sign, in disagreement with the prediction of first principles theory, and its magnitude decreases with increasing temperature much too rapidly to be explained by a power law dependence on magnetization. These behaviors could arise from the location of the Fermi level, which leaves a small region of the majority 3d t2g states unfilled, possibly favoring small internal ...


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 Mar 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

Deborah L. Schlagel

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 ...


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 ...


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 ...


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 ...


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 ...


Atomic Short-Range Order And Incipient Long-Range Order In High-Entropy Alloys, Prashant Signh, Andrei V. Smirnov, Duane D. Johnson Jun 2015

Atomic Short-Range Order And Incipient Long-Range Order In High-Entropy Alloys, Prashant Signh, Andrei V. Smirnov, Duane D. Johnson

Ames Laboratory Publications

Within density-functional theory, we apply an electronic-structure-based thermodynamic theory to calculate short-ranged order (SRO) in homogeneously disordered substitutional N-component alloys, and its electronic origin. Using the geometric properties of an (N−1) simplex that describes the Gibbs (compositional) space, we derive the analytic transform of the SRO eigenvectors that provides a unique description of high-temperature SRO in N-component alloys and the incipient low-temperature long-range order. We apply the electronic-based thermodynamic theory and the new general analysis to ternaries (A1 Cu-Ni-Zn and A2 Nb-Al-Ti) for validation, and then to quinary Al-Co-Cr-Fe-Ni high-entropy alloys for predictive assessment.


Specific Heat Investigation For Line Nodes In Heavily Overdoped Ba1−Xkxfe2as2, J. S. Kim, G. R. Stewart, Yong Liu, Thomas A. Lograsso Jun 2015

Specific Heat Investigation For Line Nodes In Heavily Overdoped Ba1−Xkxfe2as2, J. S. Kim, G. R. Stewart, Yong Liu, Thomas A. Lograsso

Ames Laboratory Publications

Previous research has found that the pairing symmetry in the iron-based superconductor Ba1−xKxFe2As2 changes from nodeless s wave near optimally doped, x≈0.4−0.55 and Tc>30K, to nodal (either d wave or s wave) at the pure end point, x=1 and Tc<4K. Intense theoretical interest has been focused on this possibility of changing pairing symmetry, where in the transition region both order parameters would be present and time-reversal symmetry would be broken. Here we report specific heat measurements in zero and applied magnetic fields down to 0.4 K of three individual single crystals, free of low-temperature magnetic anomalies, of heavily overdoped Ba1−xKxFe2As2,x=0.91, 0.88, and 0.81. The values for Tcmid are 5.6, 7.2, and 13 K and for Hc2≈4.5, 6, and 20 T, respectively. The data can be analyzed in a two-gap scenario, Δ2/Δ1≈4, with the magnetic field dependence of γ(=C/TasT→0) showing an anisotropic “S-shaped” behavior vs H, with the suppression of the lower gap by 1 T and γ≈H1/2 overall. Although such a nonlinear γ vs H is consistent with deep minima or nodes in the gap structure, it is not clear evidence for one, or both, of the gaps being nodal in these overdoped samples. Thus, following the established theoretical analysis of the specific heat of d-wave cuprate superconductors containing line nodes, we present the specific heat normalized by H1/2 plotted vs T/H1/2 of these heavily overdoped Ba1−xKxFe2As2 samples which—thanks to the absence of magnetic impurities in our sample—convincingly shows the expected scaling for line node behavior for the larger gap for all three compositions. There is, however, no clear observation of the nodal behavior C∝αT2 in zero field at low temperatures, with α≤2mJ/molK3 being consistent with the data. This, together with the scaling, leaves open the possibility of extreme anisotropy in a nodeless larger gap, Δ2, such that the scaling works for fields above 0.25–0.5 T (0.2–0.4 K in temperature units), where this is an estimate for the size of the deep minima in the Δ2∼20−25K gap. Therefore, the location of the change from nodeless to nodal gaps between optimally doped and heavily overdoped Ba1−xKxFe2As2 based on the present work may be closer to the KFe2As2 end point than x=0.91.


Anisotropic Thermal Expansion In Molecular Solids: Theory And Experiment On Libh4, Nikolai A. Zarkevich, E. H. Majzoub, Duane D. Johnson Mar 2015

Anisotropic Thermal Expansion In Molecular Solids: Theory And Experiment On Libh4, Nikolai A. Zarkevich, E. H. Majzoub, Duane D. Johnson

Nikolai A. Zarkevich

We propose a reliable and efficient computational method for predicting elastic and thermal expansion properties in crystals, particularly complex anisotropic molecular solids, and we apply it to the room-temperature orthorhombic Pnmaphase of LiBH4. Using density-functional theory, we find thermal expansion coefficients at finite temperature, and we confirm them by temperature-dependent, in situ x-ray diffraction measurements. We also consider the effects of volume and pressure, as well as energy barriers for BH4− rotations and collective motions. Our combined study validates the theory and provides a better understanding of the structural behavior of LiBH4.


Shape-Memory Transformations Of Niti: Minimum-Energy Pathways Between Austenite, Martensites, And Kinetically Limited Intermediate States, Nikolai A. Zarkevich, Duane D. Johnson Mar 2015

Shape-Memory Transformations Of Niti: Minimum-Energy Pathways Between Austenite, Martensites, And Kinetically Limited Intermediate States, Nikolai A. Zarkevich, Duane D. Johnson

Nikolai A. Zarkevich

NiTi is the most used shape-memory alloy; nonetheless, a lack of understanding remains regarding the associated structures and transitions, including their barriers. Using a generalized solid-state nudged elastic band method implemented via density-functional theory, we detail the structural transformations in NiTi relevant to shape memory: those between a body-centered orthorhombic (bco) ground state and a newly identified stable austenite (“glassy” B2-like) structure, including energy barriers (hysteresis) and intermediate structures (observed as a kinetically limited Rphase), and between martensite variants (bco orientations). All results are in good agreement with available experiment. We contrast the austenite results to those from the often-assumed ...


Nudged-Elastic Band Method With Two Climbing Images: Finding Transition States In Complex Energy Landscapes, Nikolai A. Zarkevich, Duane D. Johnson Mar 2015

Nudged-Elastic Band Method With Two Climbing Images: Finding Transition States In Complex Energy Landscapes, Nikolai A. Zarkevich, Duane D. Johnson

Nikolai A. Zarkevich

The nudged-elastic band (NEB) method is modified with concomitant two climbing images (C2-NEB) to find a transition state (TS) in complex energy landscapes, such as those with a serpentine minimal energy path (MEP). If a single climbing image (C1-NEB) successfully finds the TS, then C2-NEB finds it too. However, improved stability of C2-NEB makes it suitable for more complex cases, where C1-NEB misses the TS because the MEP and NEB directions near the saddle point are different. Generally, C2-NEB not only finds the TS, but also guarantees, by construction, that the climbing images approach it from the opposite sides along ...


Stable Atomic Structure Of Niti Austenite, Nikolai A. Zarkevich, Duane D. Johnson Mar 2015

Stable Atomic Structure Of Niti Austenite, Nikolai A. Zarkevich, Duane D. Johnson

Nikolai A. Zarkevich

Nitinol (NiTi), the most widely used shape-memory alloy, exhibits an austenite phase that has yet to be identified. The usually assumed austenitic structure is cubic B2, which has imaginary phonon modes, hence it is unstable. We suggest a stable austenitic structure that “on average” has B2 symmetry (observed by x-ray and neutron diffraction), but it exhibitsfinite atomic displacements from the ideal B2 sites. The proposed structure has a phonon spectrum that agrees with that from neutron scattering, has diffraction spectra in agreement with x-ray diffraction, and has an energy relative to the ground state that agrees with calorimetry data.


Platinum Nanoparticle During Electrochemical Hydrogen Evolution: Adsorbate Distribution, Active Reaction Species, And Size Effect, Teck L. Tan, Lin-Lin Wang, Jia Zhang, Duane D. Johnson, Kewu Bai Mar 2015

Platinum Nanoparticle During Electrochemical Hydrogen Evolution: Adsorbate Distribution, Active Reaction Species, And Size Effect, Teck L. Tan, Lin-Lin Wang, Jia Zhang, Duane D. Johnson, Kewu Bai

Ames Laboratory Publications

For small Pt nanoparticles (NPs), catalytic activity is, as observed, adversely affected by size in the 1–3 nm range. We elucidate, via first-principles-based thermodynamics, the operation H* distribution and cyclic voltammetry (CV) during the hydrogen evolution reaction (HER) across the electrochemical potential, including the underpotential region (U ≤ 0) that is difficult to assess in experiment. We consider multiple adsorption sites on a 1 nm Pt NP model and show that the characteristic CV peaks from different H* species correspond well to experiment. We next quantify the activity contribution from each H* species to explain the adverse effect of size ...


Anomalous Magneto-Structural Behavior Of Mnbi Explained: A Path Towards An Improved Permanent Magnet, Nikolai A. Zarkevich, Linlin Wang, Duane D. Johnson Feb 2015

Anomalous Magneto-Structural Behavior Of Mnbi Explained: A Path Towards An Improved Permanent Magnet, Nikolai A. Zarkevich, Linlin Wang, Duane D. Johnson

Nikolai A. Zarkevich

Low-temperature MnBi (hexagonal NiAs phase) exhibits anomalies in the lattice constants (a, c) and bulk elastic modulus (B) below 100 K, spin reorientation and magnetic susceptibilitymaximum near 90 K, and, importantly for high-temperature magnetic applications, an increasingcoercivity (unique to MnBi) above 180  K. We calculate the total energy and magneto-anisotropyenergy (MAE) versus (a, c) using DFT+U methods. We reproduce and explain all the above anomalies. We predict that coercivity and MAE increase due to increasing a, suggesting means to improve MnBi permanent magnets.


Reply To “Comment On 'Mechanochemical Continuum Modeling Of Nanovoid Nucleation And Growth In Reacting Nanoparticles'″, Valery I. Levitas, Hamed Attariani Feb 2015

Reply To “Comment On 'Mechanochemical Continuum Modeling Of Nanovoid Nucleation And Growth In Reacting Nanoparticles'″, Valery I. Levitas, Hamed Attariani

Hamed Attariani

The problem of the mechanisms of void nucleation in nanoparticles and related problem of the stresses and their relaxation in nanoparticles are of significant importance and far from being resolved. It is not surprising that there are several approaches with different and, in the given case, opposite assumptions. Thus, in ref 1, surface tension is neglected, and volumetric strain due to oxidation is introduced in the oxide shell. It is stated that for linear lattice mismatch α ≥ 0.3 large tensile stresses in a metal core are sufficient for void nucleation.


Mechanochemical Continuum Modeling Of Nanovoid Nucleation And Growth In Reacting Nanoparticles, Valery I. Levitas, Hamed Attariani Feb 2015

Mechanochemical Continuum Modeling Of Nanovoid Nucleation And Growth In Reacting Nanoparticles, Valery I. Levitas, Hamed Attariani

Hamed Attariani

Hollow nanoparticles (NPs) are produced by void nucleation and growth during chemical reactions. However, there is no proper understanding of nucleation and growth mechanisms and their predictive modeling. Models based on the Kirkendall effect predict the process time, which is larger by orders of magnitude than in experiment. This is why some works propose that a large tensile pressure in the core causes void nucleation. Here, a continuum-mechanics approach for nucleation and growth of a nanovoid in reacting NPs based on the Kirkendall effect is developed. In contrast to previous approaches, void nucleation and the effects of stresses are treated ...


Formation Of Irregular Al Islands By Room-Temperature Deposition On Nial(110), Dapeng Jing, Yong Han, Bariş Ünal, J W. Evans, Patricia A. Thiel Feb 2015

Formation Of Irregular Al Islands By Room-Temperature Deposition On Nial(110), Dapeng Jing, Yong Han, Bariş Ünal, J W. Evans, Patricia A. Thiel

Patricia A. Thiel

STM studies reveal that irregular non-equilibrium two-dimensional Al islands form during deposition of Al on NiAl(110) at 300 K. These structures reflect the multiple adsorption sites and diffusion paths available for Al adatoms on the binary alloy surface, as well as the details of inhibited edge diffusion and detachment-attachment kinetics of Al adatoms for numerous distinct step edge configurations. We attempt to capture these features by multi-site lattice-gas modeling incorporating DFT energetics for adatoms both at adsorption sites and transition states. This formulation enables description and elucidation of the observed island growth shapes.


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 2015

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

Patricia A. Thiel

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.