bluuebunny/crossref_metadata_2025 huggingface dataset:
Electronic Structure and Hydrogen Desorption in NaAlH4
Authors: Li S. , Jena P. , Araujo C. M. , Ahuja R.
Published: April 2011

First principles calculations based on gradient corrected density functional theory are carried out to understand the electronic structure and mechanisms responsible for desorption of hydrogen from Ti doped and vacancy containing sodium-alanate (NaAlH4). The energy necessary to remove a hydrogen atom from Ti doped NaAlH4 is significantly smaller than that from pristine NaAlH4 irrespective of whether Ti substitutes the Na or the Al site. However, the presence of Na and Al vacancies is shown to play an even more important role: The removal of hydrogen associated with both Na and Al vacancies is found to be exothermic. It is suggested that this role of vacancies can be exploited in the design and synthesis of complex light metal hydrides suitable for hydrogen storage.
Photoelectrochemical Properties of Highly-ordered Titania Nanotube-arrays
Authors: Paulose Maggie , Varghese Oomman K. , Shankar Karthik , Mor Gopal K. , Grimes Craig A.
Published: April 2011

We report on non-particulate titania photoelectrodes with a unique highly-ordered nanotube-array architecture prepared by an anodization process that enables precise control over array dimensions. Under 320–400 nm illumination titania nanotube-array photoanodes, pore size 110 nm, wall thickness 20 nm, and 6 μm length, generate hydrogen by water photoelectrolysis at a normalized rate of 80 mL/W•hr, to date the most efficient titania-based photoelectrochemical device, with a conversion efficiency of 12.25%. The highly-ordered nanotubular architecture allows for superior charge separation and charge transport, with a calculated quantum efficiency of nearly 100% for incident photons with energies larger than the titania bandgap.
STM Image Simulation: Effect of the Number of Tunneling States and the Isosurface Value
Authors: Radilla Juan , Galván Marcelo , Trinidad-Reyes Yolanda , Batina Nikola
Published: April 2011

In this work, Scanning Tunneling Microscopy (STM) images were simulated for the organic molecule 3,4,9,10 perylenetetracarboxylic dianhydride (PTCDA) to study the effect of the number of tunneling states and the integrated Local Density of States (LDOS) isosurface value. Local Density Approximation of Density Functional Theory (DFT-LDA) calculations were performed to achieve the simulated images under the Tersoff and Hamann approximation. The number of tunneling states has a strong effect on the image appearance of the patterns. Intermediate isosurface values for the integrated LDOS produce good resolution and matching in relation to the experimental STM image. Both parameters seem to be of significant importance for STM image simulation.
Ballistic Electron Emission Luminescence of InAs Quantum Dots Embedded in a GaAs/AlxGa1−xAs Heterostructure
Authors: Yi Wei , Appelbaum Ian , Russell Kasey J. , Narayanamurti Venkatesh , Hanson Micah P. , Gossard Arthur C.
Published: April 2011

Ballistic electron emission luminescence (BEEL) is a further development of ballistic electron emission microscopy (BEEM) combining three-terminal hot electron injection and interband radiative recombination in direct-gap semiconductor materials. By using a planar tunnel-junction emitter rather than a STM tip, a spectrographic analysis of the induced electroluminescence can be performed with the help of much higher current injection level. We demonstrate the operational principle of BEEL in a GaAs/AlxGa1−xAs heterostructure with a layer of InAs quantum dots (QDs) as the optical active layer. The wavelength-resolved BEEL spectra from planar tunnel-junction devices disclose the QD luminescence as a peak near 1.34 eV accompanied with a linear quantum-confined Stark shift. At higher collector voltage, luminescence from bulk states of GaAs peaked at 1.48 eV is observed. The spectrally integrated BEEL intensity as a function of collector voltage fits well with the results from STM tip injection, which is measured in a single-photon-counting mode. Measurement of ballistic electron current spectroscopy is made possible by freezing out the thermionic leakage current at low temperatures. Our results indicate that it is feasible to simultaneously acquire topographic, electronic and photonic information of buried light-emitting semiconductor heterostructures.
Scanning Probe Recognition Microscopy Investigation of the Elastic Properties of Tissue Scaffolding
Authors: Chen Q. , Fan Y. , Ayres V. M. , Udpa L. , Schindler M. S. , Rice A. F.
Published: April 2011

Scanning Probe Recognition Microscopy is a new scanning probe capability under development within our group to reliably return to and directly interact with a specific nanoscale feature of interest, without the use of a zoom box with its thermal drift and local origin difficulties. It is a recognition-driven and learning approach, made possible through combining SPM piezoelectric implementation with on-line image processing and dynamically adaptive learning algorithms. Segmentation plus a recognized pattern is implemented within a scan plan and used to guide the tip in a recognition-driven return to a specific site.

The specific application focus of our group is on the development of Scanning Probe Recognition Microscopy for nanobiological investigations. In the present work, Scanning Probe Recognition Microscopy is used in a direct investigation of the surface and elastic properties along individual tubules within a tissue scaffolding matrix. Elastic properties are indicated as important influences on actin polymerization and consequent cell pseudopodia extension and contraction.
Direct observation of polar nanostructures in PLZT ceramics for electrooptic applications
Authors: Shvartsman V. V. , Orlova A. , Kiselev D. , Sternberg A. , Kholkin A. L.
Published: April 2011

Transparent Pb1-yLay(Zr1-xTix)1-y/4O3(PLZT, y=0.0975, x=0.35) ceramics prepared via hot pressing techniques were studied via piezoelectric force microscopy (PFM). Clear piezoelectric contrast is observed in a cubic relaxor phase indicating spatial distribution of polarization with an average cluster size of about 50 nm. The irregular polarization pattern is associated with the formation of a glassy state, where random electric and stress fields are responsible for the disruption of the long-range ferroelectric order. Local poling of the ceramics resulted in the formation of a stable micron-size domain that could be continuously switched under varying dc bias (local hysteresis loop). The applied voltage of 10–12 Volts was enough to switch relaxor into the ferroelectric state. Fractal and correlation analysis of the observed images is presented. The fractal dimension close to 1.55 is consistent with the random distribution of charge defects in this material. The nature of the observed phenomena is discussed based on the current understanding of relaxor state in PLZT and possible effect of PFM instrumentation.
Radiation Induced Subsurface Charging in the Buried Oxide Layer in SIMOX
Authors: Stevens-Kalceff M. A. , Mickle S.
Published: April 2011

Kelvin Probe Microscopy has been used to characterize the magnitude and spatial distribution of reproducible characteristic residual potential in electron beam irradiated silicon on insulator specimens (SIMOX). Focussed electron beam irradiation produces trapped charge within the insulating buried oxide layer which produces highly localized electric fields. The charging processes are dynamic, localized, and dependent on pre-existing and irradiation induced defect concentrations. The characteristic experimental surface potential distributions are compared with calculated model surface potential distributions. This work demonstrates that proximal probe methods which are usually considered to be surface analysis techniques, can be used to investigate subsurface properties and give insight into subsurface charging processes.
Scanning Conductance Microscopy and High Frequency Scanning Gate Microscopy of Carbon Nanotubes and Polyethylene based Nanofibers
Authors: Staii Cristian , Shao Rui , Pinto Nicholas J. , Bonnell Dawn A. , Johnson Alan T.
Published: April 2011

We present two new approaches that significantly enhance the analytic power of Scanning Conductance Microscopy (SCM) and Scanning Gate Microscopy (SGM). First, we present a quantitative model that explains the phase shifts observed in SCM, by considering the change in the total capacitance of the tip-sample-substrate system. We show excellent agreement with data on samples of (conducting) single wall nanotubes and insulating polyethylene oxide (PEO) nanofibers. This model is also used to determine the dielectric constant of PEO nanofibers, a general approach that can be extended to other dielectric nanowires. Second, we extend the SGM to frequencies up to 15MHz, and use it to image changes in the impedance of carbon nanotube field effect transistor (CNFET) circuits induced by the SGM-tip gate. We show that these measurements are consistent with a simple RC parallel circuit model of the CNFET, with a time constant of 0.3 μs.
In Situ Quantitative Plasmon Spectroscopic Determination and Imaging of Multiple SolidState Properties at the Nanoscale: a New Capability for Material Research
Authors: Oleshko Vladimir P. , Howe James M.
Published: April 2011

Measuring material properties is critical to understanding the behavior of contemporary nanostructured materials. In this paper, we show that as a consequence of the universal binding energy relation (UBER), universal features and strong scaling correlations exist between the volume plasmon energy and cohesive energy, valence electron density, elastic constants and hardness of various materials with metallic and covalent bonding. Based on these relations, we propose novel techniques that allow direct measurement and imaging of material properties in situ using valence electron energy-loss spectroscopy combined with energy-filtering transmission electron microscopy. This is illustrated by evaluation of elastic and cohesive properties of individual metastable nanoprecipitates in structural alloys and hardness of diesel-engine soot particles. The results demonstrate that new plasmon spectro-microscopic techniques have the potential to determine quantitatively and image multiple solid-state properties at the nanoscale, establishing a new capability for material research.
Surface Chemistry of Combustion-Synthesized Iron Oxide Nanoparticles Determined by Electron Energy-Loss Spectroscopy
Authors: Leppert V.J. , Pinkerton K. E. , Jasinski J.
Published: April 2011

Particulate matter (PM) that is composed of a complex mixture of organic compounds, soot, transition metals, sulfates, nitrates, and other trace elements has been associated with a range of adverse health effects. The effect of the separate components of PM and their interaction with each other is important in order to understand the origins of toxicity in these materials. Iron and soot are common environmental contaminants occurring in ultrafine particulate matter in the air, and have been experimentally generated, in a manner simulating high-temperature, industrial processes, through the combustion of iron pentacarbonyl in a mixture of acetylene and ethylene. Nanoprobe electron energy-loss spectroscopy (EELS) in the transmission electron microscope was used to collect Fe L3:L2 white line-intensity ratios from the interiors and surfaces of iron oxide nanoparticles present in samples generated under low and high soot conditions. This ratio is sensitive to any change in the iron oxidation state, and indicated that iron oxide nanoparticles co-generated with large quantities of soot during combustion processes show a subsurface layer (of thickness 2–3 nm) of decreased iron oxidation state. A quantitative analysis of the iron L32 EELS ionization edge measured in nanoprobe mode indicated that in these particles, the oxidation state of iron at the surface is decreased from Fe3+ to Fe2+, possibly explaining the toxicity of these materials in the respiratory tract. The application of EELS to understanding the nanoscale distribution of the various components and their chemical interactions in these samples, and correlation of these results with acute respiratory toxicity studies of the laboratory-generated particulate matter, are discussed.
On the Effect of Local Grain-Boundary Chemistry on the Macroscopic Mechanical Properties of a High Purity Y2O3-Al2O3-Containing Silicon Nitride Ceramic
Authors: Ziegler A. , McNaney J.M. , Hoffmann M. J. , Ritchie R. O.
Published: April 2011

The effects of grain-boundary chemistry on the mechanical properties of a high-purity silicon nitride ceramics were investigated, with specific emphasis on the role of oxygen. Variations in the grain-boundary oxygen content, through control of oxidizing heat treatments and sintering additives, was found to result in a transition in fracture mechanism from transgranular to intergranular fracture, with an associated increase in fracture toughness. This phenomenon is correlated to an oxygen-induced change in grain-boundary chemistry that appears to affect fracture by “weakening” the interface, facilitating debonding and crack advance along the boundaries, thereby enhancing the toughness by grain bridging. It is concluded that if the oxygen content in the thin grain-boundary films exceeds a lower limit, which is ∼0.87 equiv% oxygen content, then the interfacial structure and bonding characteristics favor intergranular debonding during crack propagation; otherwise, transgranular fracture ensues, with consequent low toughness.
Electron Holographic Characterization of Nano-Hetero Interface Effect in Gold Catalysts
Authors: Ichikawa S. , Akita T. , Okazaki K. , Tanaka K. , Kohyama M.
Published: April 2011

We investigated the atomic structure near the interface and the size dependence of the mean inner potential of gold in Au/TiO2 catalysts prepared by the deposition precipitation (DP) method and the vacuum evaporation (VE) method using high resolution electron microscopy (HREM) and electron holography. The TiO2 supports prepared by DP method and VE method are considered to have oxygen-rich surfaces and titanium-rich surfaces respectively. In case of the Au/TiO2 catalyst prepared by DP method, the mean inner potential of gold increased depending on the size of the particle. When the size of the gold particle is over 5nm, the mean inner potential of gold was the same as that of bulk Au. When the size is below 5nm, the mean inner potential became to increase. It increases suddenly over 40V, particularly in case of below 2nm. It indicates that the electronic state of gold on TiO2 changes from that of bulk state as the size decreases. On the other hand, the mean inner potential of gold in Au/TiO2 catalysts prepared by VE method also increased as the size decreased, but the behavior of the size dependence is different from that of DP method. The critical size of the mean inner potential change is around 3nm. The size of the gold particle was below 3nm, the mean inner potential of gold increased gradually. The mean inner potential of VE method is less than that of DP method with the same particle size in spite of the size below 3nm. The stoichiometry at the interface between Au and TiO2 should be one of the dominant reasons for the behavior difference of the size dependence of the mean inner potential between the preparations.
Discrete Tomography of Ga and InGa Particles from HREM Image Simulation and Exit Wave Reconstruction
Authors: Jinschek J. R. , Calderon H. A. , Batenburg K. J. , Radmilovic V. , Kisielowski Ch.
Published: April 2011

Low-resolution tomography requires recording images every few degrees. As a consequence, the sample is often degraded after such a procedure. However the required input can be reduced drastically by using knowledge about the position and the number of atoms in each atomic column. This concept has been tested in the present investigation where HREM image simulation (MacTempas) together with exit wave reconstruction (FEI Trueimage) have been performed. A cubeoctahedral nanoparticle is used for the simulation with different compositions i.e., pure solid Ga and In-Ga particles. Six different zone axes ([111], [111], [001], [110], [110], [011]) have been used and the parameters of an aberration corrected microscope (200kV, Cs = 0 mm, resolution = 0.5Å). The discrete grid data were determined by constructing a channeling map from the reconstructed exit wave images. In this special case only three projections [001], [110], [110] were sufficient to find a unique volumetric reconstruction, illustrating the potential of the method. The other projections were used for checking the solution. The comparison between the projected potentials (simulated input) and the final result shows that discrete tomography reconstructs the exact position of all 309 atoms and the three-dimensional shape of the nanocrystal.
Electron Microscopy Analysis of the Central Dark Line Defect of the Human Tooth Enamel
Authors: Rodríguez-Hernández A. G. , Fernández M.E. , Carbajal-De-La-Torre G. , García-García R. , Reyes-Gasga J.
Published: April 2011

After some experimental results that indicated that HA is able to growth in an epitaxial way on the surface of OCP, it has been suggested that the central dark line (CDL) observed in the nanometric-sized grains of human tooth enamel corresponds to a one-unit-cell-thick layer of octacalcium phosphate (OCP). Based on this consideration, in this work we propose a model for CDL and we carried out the chemistry and structural analysis of the CDL with high resolution microscopy techniques such as Electron Energy Loss Spectroscopy (EELS) and Z-contrast (HAADF) with the aim of find the agreements and/or differences between the human tooth enamel HA and its CDL.
Mapping the Structure of a Hydrated Polymer Blend Using Energy-Loss Spectroscopy in the Cryo-STEM
Authors: Sousa Alioscka , Aitouchen Abdelaziz , Libera Matthew
Published: April 2011

We use electron energy-loss spectroscopy (EELS) in the cryo-STEM to determine the spatial distribution of water in a model frozen-hydrated two-phase polymer blend composed of hydrophilic poly(vinylpyrrolidone) (PVP) and hydrophobic poly(styrene) (PS). We demonstrate that it is possible to directly correlate the water spatial distribution with variations in the underlying polymer morphology. HAADF-STEM imaging of both dry and frozen-hydrated specimens shows weak contrast between the polymer phases but gives no information regarding the composition of these phases and no indication of where water might be localized. Spatially-resolved EELS spectra collected at 100 nm pixel size show that this system is composed of discrete PVP-rich domains dispersed in a continuous PS-rich matrix. The PVP-rich domains were found to be hydrated up to a level of ∼ 23 wt%. We have made our compositional maps fully quantitative, given as mass-fraction maps, by measuring the total inelastic scattering cross-sections per unit mass of water, PVP and PS. This is an important quantity which we have determined for an incident beam energy of 200 keV. Hydrated PVP gives rise to hydrogen evolution when irradiated above an electron dose of 1500 e/nm2 as evidenced from changes in the 13 eV region, and this effect gives rise to dose-limited resolution in these experiments.
In situ transmission electron microscopy of nano-sized metal clusters
Authors: De Hosson Jeff Th. M. , Palasantzas George , Vystavel Tomas , Koch Siete
Published: April 2011

The paper concentrates on in situ transmission electron microscopy of nano-sized Mo and Nb clusters. In particular, this contribution presents challenges to control the microstructure in nano-structured materials via a relatively new approach, i.e. using a so-called nanocluster source. An important aspect is that the cluster size distribution is monodisperse and that the kinetic energy of the clusters during deposition can be varied. The deposited Mo clusters with sizes 5 nm or larger show a body-centered crystal (bcc) structure. The cubic clusters are self-assembled from smaller ones and forming distorted cubes of typical size 7.8 nm or larger. With reducing cluster size to ≤3 nm, the face centered crystal (fcc) structure appears due to dominance of surface energy minimization, while self-assembly into large cubes with sizes up to 20 nm is still observed. In situ TEM annealing leads to cluster coalescence at temperatures ∼800 °C, with the crystal habit changing to rhombic dodecahedron for isolated clusters, while large cubes change to faceted polyhedra. In situ TEM annealing studies on Nb clusters showed that cluster coalescence events were not observed even at rather elevated temperatures because of the formation of oxides.
Neutron Spectroscopy of Carbon nano-Materials
Authors: Kolesnikov Alexander I. , Loong Chun-K. , Moravsky Alexander P. , Loutfy Raouf O. , Burnham Christian J.
Published: April 2011

Vibrational spectra for single wall carbon nanotubes, double wall carbon nanotubes, single wall carbon nanohorns and C60-peapods have been measured with inelastic neutron scattering in a wide range of energy transfer, 5–220 meV. A decrease in intensity around 75–100 meV and the appearance of two peaks around 120–125 meV and 150 meV in the double wall nanotubes and C60-peapods spectra compared to single wall carbon nanotubes and nanohorns were observed. These findings indicate the possibility of strong interaction between the walls of the double wall carbon nanotube, and between C60 molecules and carbon nanotube of the peapod. Alternatively, a possible contamination of the samples by hydrogen (even in microscopic quantities) covalently bonded to the carbon also can account for the observed phenomena.
An Experimental Evaluation of the Constant β Relating the Contact Stiffness to the Contact Area in Nanoindentation
Authors: Strader Jeremy H. , Shim Sanghoon , Bei Hongbin , Oliver W. C. , Pharr G. M.
Published: April 2011

Measurements of mechanical properties by nanoindentation with triangular pyramidal indenters like the Berkovich rely heavily upon the relationship between the contact stiffness, S, the contact area, A, and the reduced elastic modulus, Er. This relationship is often written in the form S = 2βEr(A/π)1/2, where β is a constant that depends on the geometry of the indenter. Although the most common values for β used in experimental measurements are 1.000 and 1.034, various theoretical analyses have yielded values as small as 1.00 or as large as 1.2, depending on the assumptions made to model the deformation. Here, we explore the most appropriate value of β by performing careful experiments in fused quartz with thin gold coatings applied to the surface to reveal the actual contact area when observed in the scanning electron microscope. Experiments were performed not only with the Berkovich indenter, but with five other three-sided pyramidal indenters with centerline-to-face angles ranging from 35.3° (cube corner) to 85°. Results are discussed as they apply to obtaining accurate measurements of mechanical properties by nanoindentation.
Mechanisms Underlying Hardness Numbers
Authors: Gilman John J.
Published: April 2011

Relationships of indentation hardness numbers to to other physical properties are demonstrated. They differ depending on the type of chemical bonding; metals, alloys ionic, covalent, and metal-metalloid. The properties are: shear modulus; ionic charge; band-gap density; polarizability; and formation energy, respectively. In each case the rationale is provided. The concept of a “bonding Modulus” is introduced. It is concluded that the conventional wisdom that hardness is a purely empirical property does not hold. Phase transformations and indentation hardness are connected broadly.
Dislocation Nucleation at a Surface Step by a Multiscale Approach
Authors: Li Chengzhi , Segall Darren , Xu Guanshui
Published: April 2011

Nucleation of a <111> screw dislocation at a step from a {112} surface of tantalum has been studied by a multiscale approach. The profile of the dislocation, represented by the relative displacement between two adjacent atomic layers along the slip plane, is solved based on the variational boundary integral formulation of the Peierls-Nabarro dislocation model, in which the interatomic potential is incorporated based on atomic calculations of generalized stacking fault energies. The results show that the atomic scale step significantly facilitates dislocation nucleation from the surface. The results from all atom calculations are also presented to corroborate the results obtained by this multiscale approach.
Determination of Plastic Properties of Polycrystalline Metallic Materials by Nanoindentation – Experiments and Finite Element Simulations
Authors: Durst Karsten , Backes Björn , Göken Mathias
Published: April 2011

The determination of plastic properties of metallic materials by nanoindentation requires the analysis of the indentation process and the evaluation methods. Particular effects on the nanoscale, like the indentation size effect or piling up of the material around the indentation, need to be considered. Nanoindentation experiments were performed on conventional grain sized (CG) as well as on ultrafine-grained (UFG) copper and brass. The indentation experiments were complemented with finite element simulations using the monotonic stress-strain curve as input data. All indentation tests were carried out using cube-corner and Berkovich geometry and thus different amount of plastic strain was applied to the material, according to Tabors theory. We find an excellent agreement between simulations and experiments for the UFG materials from which a representative strain of εB ≈ 0.1 and εcc ≈ 0.2 is determined. With these data, the slope of the stress-strain curve is predicted for all materials down to an indentation depth of 800 nm.
Nano-Mechanical Investigation of the Byssal Cuticle, a Protective Coating of a Bio-Elastomer
Authors: Holten-Andersen Niels , Slack Nelle , Zok Frank , Herbert Waite J.
Published: April 2011

The mechanical properties of the mussel byssal thread have been investigated via nano-indentation, with the emphasis on the differences between the cuticle and the fibrous interior. The cuticle hardness was found to be 30–40% higher than that of the underlying fibrous interior. In contrast, the Young's moduli in the two regions were virtually identical to one another. Elemental analysis via energy dispersive spectroscopy indicated surprisingly high levels of Al and Br in the cuticle considering the low amounts found in seawater. A potential role of Al in byssal thread mechanics is discussed in light of the unique capability of the cuticle to accommodate strains of 70% by the underlying fibrils in the core without delamination.
On the Measurement of Creep by Nanoindentation with Continuous Stiffness Techniques
Authors: Rar Andrei , Sohn Sangjoon , Oliver Warren C. , Goldsby David L. , Tullis Terry E. , Pharr George M.
Published: April 2011

Measurement of material creep parameters by means of nanoindentation using continuous stiffness techniques avoids the problems associated with thermal drift that often plague creep measurements based on the time dependence of the indentation depth alone [1, 2]. Problems with thermal drift are negligible from a practical point of view during continuous stiffness measurements because the contact stiffness can be measured over a short time period, typically less than one second, during which time the displacements due to thermal drift are minimal. Determination of the time dependence of the indentation depth from the stiffness data relies on the well-known relation between contact stiffness and the square root of the contact area. For pyramidal indenters, the true indentation contact depth must be proportional to the contact stiffness, leading to the assumption that indentation depth is also proportional to the contact stiffness. In this study, we critically examine this assumption using data obtained from experiments on a relatively soft material, epoxy, and a relatively hard material, fused quartz. The results show that just after initial load application, the change in contact area may be different than that expected from the change in indentation depth. One possible explanation for the observed behavior is examined by finite element modeling.
On the Measurement of Material Creep Parameters by Nanoindentation
Authors: LaManna J. A. , Oliver W. C. , Pharr G. M.
Published: April 2011

Previous studies of how material creep parameters can be measured by nanoindentation testing have focused mostly on measurement of the stress exponent for creep, n, and the activation energy, Qc. However, a more complete characterization requires that the material constant A in the uniaxial creep equation εu =Aσn (where εu is the uniaxial strain rate and σ is the uniaxial stress) also be evaluated. Here, we begin to address this issue by performing simple nanoindentation creep experiments in amorphous selenium at temperatures above and below the glass transition. At 35°C and above, the material exhibits a simple linear viscous creep behavior that is load history independent. This allows the parameter A to be determined from the indentation load-displacement-time data by means of an analytical solution. To examine the validity of the approach, values of the parameter A measured in nanoindentation tests are compared to independent measurements obtained in uniaxial tension creep experiments.
Viscoelastic Characterization of Polymers Using Dynamic Instrumented Indentation
Authors: White C. C. , Drzal P. L. , VanLandingham M. R.
Published: April 2011

Dynamic nanoindentation was performed on a poly(methyl methacrylate, PMMA), and two different poly(dimethyl siloxane, PDMS) samples having different crosslink densities. Comparison was made between dynamic nanoindentation and rheological instrumentation measurements in the glassy and rubbery plateau regions of polymeric materials. Excellent agreement between bulk rheological data and dynamic nanoindentation data was observed for the two glassy materials and the less compliant of the two PDMS samples. Results were divergent for the more compliant PDMS sample.
Spherical Indentation Creep Following Ramp Loading
Authors: Oyen Michelle L.
Published: April 2011

Depth-sensing indentation testing is a common way to characterize the mechanical behavior of stiff, time-independent materials but presents both experimental and analytical challenges for compliant, time-dependent materials. Many of these experimental challenges can be overcome by using a spherical indenter tip with a radius substantially larger than the indentation depth, thus restricting deformation to viscoelastic (and not plastic) modes in glassy polymers and permitting large loads and contact stiffness to be generated in compliant elastomers. Elastic-viscoelastic correspondence was used to generate spherical indenter solutions for a number of indentation testing protocols including creep following loading at a constant rate and a multiple ramp-and-hold protocol to measure creep response at several loads (and depths) within the same test. The ramp-creep solution was recast as a modification to a step-load creep solution with a finite loading rate correction factor that is a dimensionless function of the ratio of experimental ramp time to the material time constant. Creep tests were performed with different loading rates and different peak load levels on glassy and rubbery polymeric materials. Experimental data are fit to the spherical indentation solutions to obtain elastic modulus and time-constants, and good agreement is found between the results and known modulus values. Emphasis is given to the use of multiple experiments (or multiple levels within a single experiment) to test the a priori assumption of linear viscoelastic material behavior used in the modeling.
Multiscale Modeling of Two Dimensional Rough Surface Contacts
Authors: Luan Binquan , Hyun Sangil , Robbins Mark O. , Bernstein Noam
Published: April 2011

A hybrid simulation method is used to study non-adhesive contact between two-dimensional self-affine rough surfaces. We find strong scale dependence as the contact dimensions decrease to the atomic scale. Both purely elastic and plastic materials are considered, and show different trends with system size. In all cases the contact area rises linearly with applied load, but the slope is substantially different from continuum predictions. We also examine the effect of commensurability between contacting surfaces. Contact area and frictional forces can be very different for commensurate and incommensurate surfaces, particularly for plastic materials.
Friction and Wear Properties of ALD Coated MEMS
Authors: Nistorica Corina , Gory Igor , Skidmore George D. , Mantiziba Fadziso M. , Gnade Bruce E.
Published: April 2011

A comparative study of the microtribological properties of native oxide covered single crystal silicon and silicon coated with atomic layer deposited (ALD) alumina films is presented. The dry friction and wear behavior were investigated using a novel microelectromechanical system (MEMS) tribotester. The coefficient of friction for alumina coated surfaces and for silicon uncoated surfaces was monitored before and after wear. The friction versus normal load curves of uncoated silicon can be described by a Johnson-Kendall-Roberts model with pressure dependent shear strength while for the alumina coated surfaces, a linear dependence between the friction force and the normal load was found. Both uncoated silicon surfaces and alumina coated surfaces showed a decrease of the friction force with the number of sliding cycles.
Aspects of Tip Shape Characterization for Nanoindentation of Compliant Materials
Authors: VanLandingham Mark R. , Juliano Thomas F. , Hagon Matthew J.
Published: April 2011

The application of nanoindentation methods to compliant materials, such as polymeric and biological materials, often requires the use of instrumentation designed with enhanced force sensitivity, which limits its maximum force level. Because tip geometry is normally characterized using indentation of fused silica, the maximum contact depths achieved by low-force instruments during this calibration are often less than 300 nm. However, penetration into more compliant materials can be several micrometers or more. Extrapolation of tip shape data from fused silica indentation can lead to significant uncertainties in the indentation measurements for compliant materials. In this paper, atomic force microscopy (AFM) is used to provide tip shape information for a Berkovich tip and a series of conical tips. Use of AFM allows a larger range of depth to be calibrated and provides three-dimensional tip information.
A critical examination of the Berkovich vs. conical indentation based on 3D finite element calculation
Authors: Shim Sanghoon , Oliver Warren C. , Pharr George M.
Published: April 2011

Much of our understanding of the elastic-plastic contact mechanics needed to interpret nanoindentation data comes from two-dimensional, axisymmetric finite element simulations of conical indentation. In many instances, conical results adequately describe real experimental results obtained with the Berkovich triangular pyramidal indenter, particularly if the angle of the cone is chosen to give the same area-to-depth ratio as the pyramid. For example, conical finite element simulations with a cone angle of 70.3° have been found to accurately simulate experimental load-displacement curves obtained with the Berkovich indenter. There are instances, however, where conical simulations fail to capture important behavior. Here, 3D finite element simulations of Berkovich indentation of fused silica are compared to similar simulations with a 70.3° cone. It is shown that a potentially significant difference between the two indenters exists for the contact areas and contact stiffnesses. Implications for the interpretation of nanoindentation data are discussed.
Quantitative Analysis of γ Precipitates in Cyclically Deformed Ni3(Al, Ti) Single Crystals Using Magnetic Technique
Authors: Umakoshi Yukichi , Yasuda Hiroyuki Y. , Yanai Toshifumi
Published: April 2011

ABSTBACT

Ni-18at.%Al-4at.%Ti single crystals containing small γ precipitates in γ′ matrix were prepared by aging at 1073K after solution treatment from γ′ single phase region. The size, shape and volume fraction of the γ precipitates were examined during aging and deformation by means of changes in spontaneous magnetization and coercive force. Cyclic deformation behavior of these crystals was discussed through change in the morphology of γ precipitates using magnetic technique.
Transformation Behavior of TiNiPt Thin Films Fabricated Using Melt Spinning Technique
Authors: Inamura Tomonari , Takahashi Yohei , Hosoda Hideki , Wakashima Kenji , Nagase Takeshi , Nakano Takayoshi , Umakoshi Yukichi , Miyazaki Shuichi
Published: April 2011

Martensitic transformation behavior of Ti50Ni40Pt10 (TiNiPt) melt-spun ribbons were investigated where the heat treatment temperature was systematically changed from 473K to 773K. A hot-forged bulk TiNiPt material with the similar chemical composition was also tested as a comparison. θ-2θ X-ray diffraction analysis and transmission electron microscopy observation revealed that the as-spun ribbons were fully crystallized. The apparent phases of as-spun ribbons at room temperature are both B19 martensite and B2 parent phase instead of B2 single phase for the hot-forged bulk material. No precipitates were found in as-spun and heat-treated ribbons. It was revealed by differential scanning calorimetry that all the specimens exhibit one-step transformation. The martensitic transformation temperatures of the TiNiPt as-spun ribbons are 100K higher than those of the hot-forged bulk material, and the martensitic transformation temperature decreases with increasing heat treatment temperature.
Sub-nano and Nano-structures of Hydrides of LaNi5and its related Intermetallics
Authors: Akiba Etsuo , Sakaki Kouji , Nakanura Yumiko
Published: April 2011

Sub-nano and nano-structures of intermetallics such as LaNi5and its related alloys were studied by in-situ X-ray and neutron diffraction methods. From the profile shape analysis, changes in lattice strain and crystalline size during hydrogenation/dehydrogenation were estimated, while the crystal (sub-nano) structures were refined by the Rietveld method using diffraction patterns. The crystallite size of the alloys studied did not change during hydrogenation and dehydrogenation. It was found formation of remarkably dense dislocations and vacancies in the lattice of hydrides of LaNi5and related intermetallics.
Atomic, Electronic, and Magnetic Structure of Iron-Based Sigma-Phases
Authors: Korzhavyi Pavel A. , Sundman Bo , Selleby Malin , Johansson Börje
Published: April 2011

A combination of ab initio total energy calculations with Calphad approach is applied to model the site occupancy and thermodynamic properties of the Fe-Cr, Co-Cr, Fe-V, and Fe-Mo binary sigma-phases as a function of composition and temperature. For each binary sigma-phase the parameters of the model are the ab initio calculated total energies of so-called end-member compounds, which represent all the 25=32 variants of complete occupancy of each of the five crystallographically inequivalent sites by one or the other alloy component. The paramagnetic state of the sigma-phases has been taken into account within the disordered local moment approach. The Fe and Co atoms are found to retain high spin moments when they occupy high-coordination-number sites in the structure. Using our model we were able to reproduce the experimentally observed site occupancy in the FeCr sigma-phase. The calculated site occupancies in the Co-Cr, Fe-V, and Fe-Mo sigma-phases are also presented and discussed.
Characterization and Catalytic Properties of Ni3Al for Hydrogen Production from Methanol
Authors: Xu Ya , Kameoka Satoshi , Kishida Kyosuke , Demura Masahiko , Tsai An-pang , Hirano Toshiyuki
Published: April 2011

The stability of catalytic activity and selectivity of Ni3Al for methanol decomposition were studied by life test at 633 K on the alkali-leached powder samples. The characterization of the samples was carried out by X-ray diffraction, inductively coupled plasma (ICP) analysis, SEM observation, and surface area measurement. The life test showed that the alkali-leached Ni3Al exhibits a very stable activity and a high selectivity for methanol decomposition. The surface characterization after reaction suggests that the high selectivity and stable activity may be attributed to the formation of tiny particles and porous structure which increased the surface area significantly during reaction. These results indicate a possibility of Ni3Al as a catalyst for hydrogen production reaction.
Thermomechanical Treatment of a Fe3Al alloy
Authors: Konrad Joachim , Zaefferer Stefan , Schneider André , Frommeyer Georg , Raabe Dierk
Published: April 2011

A binary Fe3Al alloy is investigated with respect to hot and warm rolling behavior and microstructural as well as microtextural modifications. Rolling has been performed in the A2 and B2 order regimes. The differences in microstructure are investigated. The performed texture analysis reveals the differences in hot and warm rolling textures depending on the hot rolling temperature. On the basis of microtexture investigations by means of electron backscatter diffraction (EBSD) differences concerning orientation gradients and sub-grain structures are found. A model of combined order-related and non-order related effects is proposed explaining the observed material behavior. The results are used for process modification.
Ab initio calculation of point defect energies and atom migration profiles in varying surroundings in L12-ordered intermetallic compounds
Authors: Vogtenhuber Doris , Houserova Jana , Wolf Walter , Podloucky Raimund , Pfeiler Wolfgang , Püschl Wolfgang
Published: April 2011

Formation energies of antisite defects and vacancies were derived for the L12-ordered intermetallics Ni3Al, Ni3Ga, Pt3Ga, and Pt3In by a supercell ab initio approach. A thermodynamic treatment of point-like defects was then used for the calculation of temperature-dependent defect properties. Energy profiles for atom jumps in Ni3Al in systematically varied atomic neighborhoods were calculated by statically displacing the jumping atom or by using a nudged elastic band method. It is discussed how a kinetic Monte-Carlo model can be modified so that the jump barrier height reflects the strongest neighborhood influences.
Experimental Investigation and Thermodynamic Modeling of Phase Equilibria in the Hf-Ti-Si System
Authors: Yang Y. , Bewlay B. P. , Jackson M. R. , Chang Y. A.
Published: April 2011

Phase equilibria in ternary Hf-Ti-Si alloys were studied in the as-solidified and heat treated conditions using scanning electron microscopy, x-ray diffraction, and electron beam microprobe analysis. Selected solid-solid phase equilibria at 1350°C and a partial liquidus projection of the Hf-Ti-Si system at the metal rich end of the phase diagram were established. These data were then used to develop a thermodynamic description of the Hf-Ti-Si system using the CALPHAD (CALculation of PHAse Diagram) approach. The calculated isothermal section at 1350°C and the liquidus projection can satisfactorily account for the available experimental phase equilibria data and solidification paths. Both the calculations and the experimental data suggested that the metal-rich end of the ternary phase diagram possesses one transition reaction: L + (Hf, Ti)5Si3 → Hf(Ti)2Si + β(Hf, Ti, Si).
Sputtered Coatings Based on the Al2Au Phase
Authors: Mitterer Christian , Lenhart Helmut , Mayrhofer Paul H. , Kathrein Martin
Published: April 2011

Transition metal nitride-based, wear-resistant hard coatings on cutting tools and other substrates often lack distinct colorations allowing product differentiation and self-lubricating properties. In the present work, the possibility of achieving these objectives for sputtered coatings based on the purple-red Al2Au phase within the Al-Au system was investigated. Coatings were characterized with respect to morphology, chemical and phase composition, hardness, optical, oxidation and tribological properties. Al2Au-containing coatings were deposited with dense, fine-grained structures yielding a hardness of 4 GPa and pink coloration. The coatings were stable up to about 850°C, where the onset of oxidation occurs. Low friction coefficients against alumina were achieved between 500 and 700°C. The concept of applying Al2Au-containing coatings as a colored self-lubricating layer on top of a hard coated cemented carbide tool warrants further investigations.
Micro Fracture Toughness Testing of TiAl Based Alloys with a Fully Lamellar Structure
Authors: Takashima K. , Halford T. P. , Rudinal D. , Higo Y. , Takeyama M.
Published: April 2011

A micro-sized testing technique has been applied to investigate the fracture properties of lamellar colonies in a fully lamellar Ti-46Al-5Nb-1W alloy. Micro-sized cantilever specimens with a size ≈ 10 × 10 × 50 μm3 were prepared by focused ion beam machining. Notches with a width of 0.5 μm and a depth of 5 μm were also introduced into the micro-sized specimens by focused ion beam machining. Fracture tests were successfully completed using a mechanical testing machine for micro-sized specimens at room temperature. The fracture toughness (KQ) values obtained were in the range 1.4–7 MPam1/2. Fracture surface observations indicate that these variations are attributable to differences in local lamellar orientations ahead of the notch. These fracture toughness values are also lower than those having been previously reported in conventional samples. This may be due the absence of significant extrinsic toughening mechanisms in these micro-sized specimens. Fracture mechanisms of these alloys are also considered on the micrometer scale. The results obtained in this investigation give important and fundamental information on the development of TiAl based alloys with high fracture toughness.
Experimental Studies and Thermodynamic Simulation of Phase Transformations in γ-TiAl Based Alloys
Authors: Chladil Harald F. , Clemens Helmut , Leitner Harald , Bartels Arno , Gerling Rainer , Marketz Wilfried T. , Kozeschnik Ernst
Published: April 2011

Phase transformations and phase transition temperatures in several Nb-rich γ-TiAl based alloys were investigated experimentally and compared to thermodynamic simulations. The present study combines light-optical and scanning electron microscopy, X-ray diffraction and differential-scanning-calorimetry for the characterization of the prevailing phases and phase transformations. Thermodynamic simulation based on the CALPHAD method was used for predict phase stabilities. The results from experiments on a variety of γ-TiAl based alloys are compared to thermodynamic calculations. Finally, the influence of carbon on the transition temperatures is presented.
Phase Transformation in Orthorhombic Ti2AlNb Alloys Under Severe Deformation
Authors: Greenberg B. A. , Kazantseva N. V. , Pilugin V. P.
Published: April 2011

It was found that severe plastic deformation of orthorhombic alloys caused phase transformations of the displacement type and those associated with a change in the degree of long-range order, namely B2→ω(B82 ), B2→B19 and B2→β (BCC) (in the case of alloy with initial B2-phase structure) and O→B19→A20 (initial O-phase structure, Ti2AlNb). Unlike to ordinary metals, severe plastic deformation of the titanium aluminum intermetallics leads to decreasing of the strength of the material. The B19 and A20 phases are metastable. They are absent in the equilibrium phase diagrams of the compounds under investigation. The formation of the disordered phase states with extensive sliding and having great plasticity under severe deformation makes possible to consider severe deformation as the way for increasing of plasticity of the titanium aluminides.