bluuebunny/crossref_metadata_2025 huggingface dataset:
Correlation Between Resistivity and Yellow Luminescence Intensity of MOCVD-Grown GaN Layers
Authors: Hinoki Akihiro , Hiroyama Yuichi , Tsuchiya Tadayoshi , Yamada Tomoyuki , Iwami Masayuki , Imada Katsuhiro , Kikawa Junjiroh , Araki Tsutomu , Suzuki Akira , Nanishi Yasushi
Published: April 2011

For further improvements in AlGaN/GaN heterojunction field-effect transistor performance (HFET), it is necessary to reduce the leakage current of the GaN buffer layer. We found a correlation between the leakage current and the intensity of the yellow luminescence of GaN layers taken by UV lamp excitation. The GaN layers were grown by metal organic chemical vapor deposition on SiC substrates. When the samples were excited by a UV (365 nm) lamp, visible yellow luminescence was observed. The leakage current of the GaN buffer layer was measured after deposition of ohmic metal contact. We confirmed clear correlation between the leakage current and the luminescence intensity based from result that the samples with the larger leakage current showed the stronger luminescence intensity. This correlation gives us useful information to understand the drain-source leakage current of AlGaN/GaN HFET.
Donor-like Deep Level Defects in GaN Characterized by Double-correlation Deep Level Transient Spectroscopy
Authors: Ahoujja Mo , Hogsed M , Yeo Y. K. , Hengehold R. L.
Published: April 2011

Si doped GaN grown by molecular beam epitaxy on sapphire substrates were characterized by capacitance transient spectroscopy. Conventional deep level transient spectroscopy (DLTS) measurements displayed six deep level defects, labeled A1, A, B, C1, C, and D, with activation energy ranging from 0.20 to 0.82 eV below the conduction band. Based on the logarithmic dependence of the DLTS spectral peaks on the filling pulse width, it is deduced that the defects A, B, C, and D are concentrated in the vicinity of line dislocations. Double-correlation DLTS (DDLTS) measurements, on the other hand, showed that only defects A (0.82 eV) and D (0.22 eV) exhibited deep donor-like characteristics. Following a 1.0 MeV electron irradiation of the GaN sample, one radiation-induced peak, E, with activation energy less than 0.20 eV was observed in the DLTS spectrum. However, after annealing at 350 °C, this DLTS peak intensity was found to diminish significantly.
Implementing a Hydrogen Energy Infrastructure:Storage Options and System Design
Authors: Ogden Joan , Yang Christopher
Published: April 2011

The development of a hydrogen infrastructure has been identified as a key barrier to implementing hydrogen as for a future transportation fuel. Several recent studies of hydrogen infrastructure have assessed near-term and long-term alternatives for hydrogen supply [1-2]. In this paper, we discuss how advances in material science related to hydrogen storage could change how a future hydrogen infrastructure is designed. Using a simplified engineering/economic model for hydrogen infrastructure design and cost, we explore some potential impacts of advances in storage materials, in terms of system design, cost, energy use, and greenhouse gas emissions. We find that the characteristics of hydrogen storage play a major role in the design, cost, energy use, and CO2 emissions of hydrogen supply infrastructure.
Environmental Life Cycle Inventory of Crystalline Silicon Photovoltaic Module Production
Authors: Wild-Scholten Mariska de , Alsema Erik A.
Published: April 2011

Together with 11 European and US photovoltaic companies an extensive effort has been made to collect Life Cycle Inventory (LCI) data that represents the status of production technology for crystalline silicon modules for the year 2004. These data can be used to evaluate the environmental impacts of photovoltaic solar energy systems.

The new data covers all processes from silicon feedstock production via wafer- and cell- to module manufacturing. All commercial wafer technologies are covered, i.e multi- and mono-crystalline wafers as well as ribbon technologies. For monocrystalline silicon wafer production further improvement of the data quality is recommended.
Toxicological Profiles of Nanomaterials
Authors: Rushton Erik K , Oberdörster Günter , Finkelstein Jacob
Published: April 2011

With the passage of the National Nanoscale Initiative in 2001 there has been increasing attention and funding given to nanomaterial research. This has led to a number of new materials developed at the nanoscale (< 100 nm) level, which often possess chemical and physical properties distinct from those of their bulk materials. These unique qualities are proving to be quite useful in a number of new applications. For example, biological applications in imaging, treatment, and drug delivery are particularly promising as well as the increasing engineering potential of nanocircuitry and materials science. As the number of applications increases however, so too does the potential for human exposure to nanomaterials through a number of routes: dermal, ingestion, inhalation, and even injection. Interestingly some of the properties of these nanomaterials that make them useful in these emerging technologies are the same properties that can increase their toxic potential. This is leading to an emerging discipline – nanotoxicology, which can be defined as safety evaluation of engineered nanostructures and nanodevices. Nanotoxicology research will not only provide information for risk assessment of nanomaterials based on data for hazard identification, dose response relationships and biokinetics, but will also help to further advance the field of nanoresearch by providing information to alter undesirable nanomaterials properties. Although nanotoxicology is in its infancy, there are some preliminary studies with newly developed materials that provide some insight into potential effects, which when coupled with older studies provides some insight on how these nanomaterials impact the biological system. This presentation summarizes results of studies with nanosized particles with a focus on the respiratory system and skin as portals of entry. The ability of particles to translocate from their site of entry, their ability to elicit biological responses, and their presumed mechanisms of action will be highlighted. This will be an attempt to illustrate how pervasive these materials can be, which may or may not be detrimental. With proper toxicological assessment this potential may be harnessed leading to breakthroughs at the nanotechnology – biology interface.
Importance of Turning to Renewable Energy Resources with Hydrogen as a Promising Candidate and on-board Storage a Critical Barrier
Authors: Dillon Anne C. , Nelson Brent P. , Zhao Yufeng , Kim Yong-Hyun , Tracy C. Edwin , Zhang Shengbai B.
Published: April 2011

The majority of the world energy consumption is derived from fossil fuels. Furthermore, the United States (US) consumption of petroleum vastly exceeds its production, with the majority of petroleum being consumed in the transportation sector. The increasing dependency on foreign fuel resources in conjunction with the severe environmental impacts of a petroleum-based society dictates that alternative renewable energy resources be developed. The US Department of Energy's (DOE's) Office of Energy Efficiency and Renewable Energy and the Office of Basic Energy Sciences are currently promoting a vehicular hydrogen-based energy economy. However, none of the current on-board storage technologies are suitable for practical and safe deployment. Significant scientific advancement is therefore still required if a viable on-board storage technology is to be developed. A detailed discussion of the benefits of transitioning to a hydrogen-powered automotive fleet as well as the tremendous technical hurdles faced for the development of an on-board hydrogen storage system are provided here. A novel class of theoretically predicted nanostructured materials that could revolutionize hydrogen storage materials is also presented.
Comparative life cycle assessment of three drink delivery systems
Authors: Martino Dario , Selke Susan , Joshi Satish
Published: April 2011

This paper presents information regarding a comparative life cycle assessment (LCA), including scenario and data uncertainty, based on a hypothetical drink product for which we evaluate three main container material alternatives: (1) a polyethylene terephthalate (PET) bottle; (2) an aluminum can; and (3) a polylactide (PLA) bottle. The scope of results included energy, water use and two characterized impacts are included: global warming potential (GWP) and ozone depletion potential (ODP).

The results presented in this study demonstrate that LCI parameter uncertainty seems to have a dominant effect in the outcome of the LCA results considered. Indeed, in most cases there is overlap in the uncertainty intervals (at 95% confidence level), indicating that the three systems under study have similar indicator values, at least with regard to the environmental indicators included here.
Structural Refinement in Al-MoO3 Nanocomposites Prepared by Arrested Reactive Milling
Authors: Umbrajkar Swati , Schoenitz Mirko , Dreizin Edward L
Published: April 2011

Al-MoO3 nanocomposites were synthesized based on arrested reactive milling. Altering the milling parameters leads to different scales of refinement in the structure of the reactive nanocomposites. The objective of this work is to determine the range in which the degree of structural refinement can be changed in a controlled manner. The milling intensity was controlled by using different milling media along with varying amounts of process control agent (PCA). XRD, SEM, DSC and wire-ignition tests were performed to analyze the Al-MoO3 nanocomposites. Results indicate that there is a decrease in the crystallite size with increase in the milling intensity. However increase in milling intensity also stimulates the undesired reaction between Al and MoO3. Milling conditions resulting in the highest structural refinement and lowest ignition temperatures were identified.
The Effect of Nanopowder Attributes on Reaction Mechanism and Ignition Sensitivity of Nanothermites
Authors: Puszynski Jan A , Bulian Chris J , Swiatkiewicz Jacek J
Published: April 2011

Nanothermite composites have several properties that are not typical of conventional thermites. The nanoscale size of individual reactants is responsible for the significant differences in these properties, especially the rate of energy release and mechanism of combustion front propagation. Several thermite mixtures were investigated, including Al-Fe2O3, Al-CuO, Al-MoO3, and Al-Bi2O3. Previous studies have reported on the behavior of these mixtures during unconfined burning and on the characterization of particle attributes such as particle size, surface area, and reactive metal content. This study was focused on several other attributes, including mixing of nanoreactants in water and measurements of reaction kinetics and combustion front propagation characteristics under confined conditions. The nanoscale nature of the thermite components also has an effect on the kinetics of the reaction. Differential scanning calorimetry was used to determine activation energy of these reacting systems. Several experimental setups were used to monitor the nanothermite mixtures during combustion. The mixtures were monitored during combustion in small diameter tubes using high speed video technology and a pressure sensor system. These tests were used to characterize combustion propagation under confined conditions and to determine the effect of pressure and mixture density on propagation rate. Experiments were also performed using both a closed volume pressure cell and recoil force cell to measure the reactive energy of the mixtures.
Role of Constituent Configuration on Shock-Induced Reactions in a Ni+Al Powder Mixture
Authors: Eakins Daniel , Thadhani Naresh
Published: April 2011

Ultra-fast reactions initiated within or immediately behind the shock front in powder mixtures are of importance in the synthesis of high-pressure phases and next-generation energetic materials. Reactions in nickel and aluminum powder mixtures and the establishment of a reaction threshold have been the source of many studies over the last 20 years. Prior work has suggested that the criterion for reaction is most probably mechanochemical in nature, in which shock loading environment plays a larger role than absolute shock energy input. The mechanisms responsible for intimate mixing of fresh reactants are however still unclear. In this work we are investigating the role of particle size and morphology on the loading, mixing, and their subsequent shock-induced reaction behavior, by performing shock-compressibility experiments on equi-volumetric mixtures of nickel and aluminum powders, with variations in nickel particle size (micron and nano-scale) and shape (spherical and flake). Determination of shock states is accomplished through time-resolved in situ PVDF gauge measurements of input shock stress and shock propagation speed obtained from transit time through the thickness of powder mixture. The reaction product shock-compressibility state is also being calculated based on constant pressure approximations to allow correlation with measured states for inference of the occurrence of shock-induced chemical reactions. The results of this study suggest that powder configuration in the nickel-aluminum system can be modified to encourage or discourage reaction.
Tensile Properties of Hydroxyapatite Whisker Reinforced Polyetheretherketone
Authors: Converse Gabriel , Roeder Ryan
Published: April 2011

Polyetheretherketone (PEEK) was reinforced with 0-40 vol% hydroxyapatite (HA) whiskers using a novel powder processing and compression molding technique. A powder mixture was uniaxially pressed into a composite powder compact and compression molded into a flat composite bar using an open-channel die, such that the HA whiskers exhibited a preferred orientation along the length of the bar and tensile specimens. As expected, increased HA whisker reinforcement resulted in increased elastic modulus, but decreased ultimate tensile strength and strain- or work-to-failure. PEEK reinforced with 40 vol% HA whiskers exhibited an elastic modulus of 16-18 GPa. PEEK reinforced with 20 vol% HA whiskers had an ultimate tensile strength of 70-80 MPa. Human cortical bone exhibits an elastic modulus of 17-26 GPa and an ultimate tensile strength of 80-150 MPa in the longitudinal direction (direction of principal stress). Stiffness coefficients measured by ultrasonic wave propagation indicated a level orthotropy also similar to that of human cortical bone tissue.
Local Thermal Analysis: Study of viscoelastic properties and time dependence in Surlyn
Authors: Kulkarni Harsha Prabhakar , Mogilevsky Gregory , Mullins William , Kleinhammes Alfred , Wu Yue
Published: April 2011

Self-healing ability is a fascinating property of some materials. Structural materials with self-healing ability require a subtle combination of flow and stiffness characteristics. A vivid demonstration of self-healing is the automatic sealing of the bullet hole after bullet penetration of the target material. The ionomer Surlyn® satisfies the needs of both structural requirements and self-healing to a certain extent. The viscosity of Surlyn is very high at room temperature, a favorable property for structural applications but unfavorable for self-healing. However, when it was heated to 100°C and then cooled back down to room temperature, its viscosity remains temporarily low at room temperature for a few minutes, a favorable condition for healing. Here we report the measurement of such short-time relaxation effects on flow properties in Surlyn using an atomic force microscope (AFM)-based local thermal mechanical analyzer (LTA).
Local Mineral and Matrix Changes Associated with Bone Adaptation and Microdamage
Authors: Kohn David H. , Sahar Nadder D. , Hong Sun Ig , Golcuk Kurtulus , Morris Michael D.
Published: April 2011

Skeletal fractures represent a significant medical and economic burden for society. It is generally thought that a high incidence of musculoskeletal fatigue loading results in damage accumulation at too high of a rate to be efficiently remodeled, leading to skeletal fracture. The state of damage in bone at a given time is therefore the net result of damage and repair processes, and is dependent upon extrinsic factors such as mechanical history, but also upon intrinsic factors, such as composition of bone mineral and matrix. In this invited paper, we review investigations on the coupling of Raman spectroscopy with mechanical loading of bone, providing insight into mechanisms of ultrastructural deformation in bone at smaller scales than previously understood. We also present new data showing that in-vivo mechanical loading results in increased resistance to fatigue damage, coupled with an increase in phosphate to amide I ratio and decrease in carbonate to phosphate ratio. Taken together, the data demonstrates the ability to modulate the mechanical and chemical properties of bone via exogenous mechanical stimulation.
Fatigue Microdamage in Bovine Cortical Bone Imaged by Micro-Computed Tomography Using a Barium Sulfate Contrast Agent
Authors: Leng Huijie , Wang Xiang , Niebur Glen L , Roeder Ryan K
Published: April 2011

Accumulation of microdamage during fatigue can lead to increased fracture susceptibility in bone. Current techniques for imaging microdamage in bone are inherently destructive and two-dimensional. A non-destructive, three-dimensional technique is needed to measure the spatial density of microdamage accumulation. Therefore, the objective of this study was to image microdamage accumulation in cortical bone during fatigue using micro-computed tomography (micro-CT) with a barium sulfate (BaSO4) contrast agent. Bovine cortical bone beams were loaded under four-point bending fatigue. Two symmetric notches were machined on the tensile surface in order to generate damage at the stress concentrations during loading. Specimens were loaded to a specified number of cycles or until one notch fractured, such that the other notch exhibited accumulated microdamage just prior to fracture. Microdamage ahead of the notch was stained by precipitation of BaSO4 and imaged using micro-CT. Reconstructed images showed a distinct region of bright voxels around the notch tip or along propagating cracks due to the presence of BaSO4, which was verified by backscattered electron imaging and energy dispersive spectroscopy. The stained region exhibited a characteristic kidney shape perpendicular to the notch tip, which was correlated to principal strain contours calculated by finite element analysis. The area of stained regions was positively correlated with the number of loading cycles.
Electrochemical Dip-Pen Nanolithography of Conductive Wires
Authors: Zhang Fapei , Yamada Ryo , Tada Hirokazu
Published: April 2011

We have explored the fabrication of conductive nanowires on different types of substrates by electrochemistry-assisted DPN (E-DPN) with an atomic force microscope. Various parameters of E-DPN were examined systematically including the effects of coating methods and the types of the tips on the electrodeposition behavior. It was found that a chemically-modified Si AFM tip is very suitable for E-DPN. Platinum and polyaniline nanowires with a line width of ca.100 nm to sub- 100 nm were prepared on the metallic (Au) and semiconducting (Si) surfaces. The parameters indispensable for E-DPN of metal and polymer nanowires on an insulating SiO2 surface will also be discussed.
Synthesis of Hybrid Colloids Through the Growth of Polystyrene Latex Particles onto Methacryloxy methyl triethoxysilane - Functionalized Silica Particles
Authors: Perro Adeline , Reculusa Stéphane , Bourgeat-Lami Elodie , Duguet Etienne , Ravaine Serge
Published: April 2011

One area of particular effort recently is the use of colloidal particles as precursors in engineering new materials. Nevertheless, these particles are nearly always spheres. This places limitations on the structures that can be built, especially in making photonic-bandgap materials. Therefore, it is a great challenge to create new colloids with an original shape, i.e. different from the sphere, in a controllable manner.

We present here an original approach to create hybrid organic-inorganic colloidal particles with a perfect controlled shape. The synthetic route of these structures, which are composed of spherical silica spheres surrounded by a varying number of polystyrene beads, consists in the emulsion polymerization of styrene in presence of silica particles, which had been surface-modified by a coupling agent containing polymerizable groups.

The influence of the size of the silica particle and of the nature of the coupling agent on the resulting colloidal particles morphologies was carefully analyzed. We show in particular that the number of growing polystyrene beads varies homogeneously with the diameter of the mineral spheres. A main advantage of this technique is also that we can precisely control the reaction time, allowing us to tune the final morphology of the hybrid structures. These colloidal assemblies are original building blocks for the elaboration of new functional materials.
Production and Properties of Glass Fibre-Reinforced Polymer Composites with Nanoparticle Modified Epoxy Matrix
Authors: Wichmann Malte H.G. , Gojny Florian H. , Sumfleth Jan , Fiedler Bodo , Schulte Karl
Published: April 2011

Increasing the mechanical performance, e.g. strength, toughness and fatigue properties of composites is the objective of many ongoing research projects. Nanoparticles, e.g. carbon nanotubes (CNTs) and fumed silica provide a high potential for the reinforcement of polymers. Their size in the nanometre regime make them suitable candidates for the reinforcement of fibre reinforced polymers, as they may penetrate the reinforcing fibre-network without disturbing the fibre-arrangement.

In this work, glass fibre-reinforced epoxy composites with nanoparticle modified matrix systems were produced and investigated. GFRPs containing different volume fractions of the nanofillers were produced via resin transfer moulding. Matrix dominated mechanical properties of the GFRP laminates could be improved by the incorporation of nanoparticles. The addition of only 0.3 wt.% CNTs to the epoxy matrix increased the interlaminar shear strength from 33.4 to 38.7 MPa (+16%). Furthermore, the application of electrically conductive nanoparticles enables the production of conductive nanocomposites. This offers a high potential for antistatic applications and the implementation of functional properties in the composite structures. The effects of different filler types and volume fractions on the electrical properties of the GFRPs were investigated. GFRPs containing 0.3 wt.% of CNTs, for example, exhibit an anisotropic electrical conductivity. Furthermore, an electrical field was applied to the composites during curing. The effects on the resulting electrical and mechanical properties are discussed.
Self-assembled Nanoline Template for Growth of Nanoparticles and Nanowires on Si(001)
Authors: Owen James Hugh Gervase , Miki Kazushi
Published: April 2011

Long, straight self-assembled nanolines of Bismuth are used as an atomic-scale non-lithographic template for the deposition of different metals. The Bi nanolines are 15 Å wide and can grow to lengths exceeding 1μm, limited only by the substrate terraces. We describe the method of formation of these templates, and demonstrate strongly preferential adsorption of different metals. We observe both wetting behaviour, in which elongated metal islands form nanowires along the template, and nonwetting behaviour, in which arrays of metal clusters of uniform size are produced.
Metal Organic Chemical Vapor Deposition Growth of GaN and GaMnN Multifunctional Nanostructures
Authors: Gupta Shalini , Kang Hun , Kane Matthew , Fenwick William E , Li Nola , Strassburg Martin , Asghar Ali , Dietz Nikolaus , Ferguson Ian T
Published: April 2011

Quantum dots (QDs) have been shown to improve the efficiency and optical properties of opto- electronic devices compared to two dimensional quantum wells in the active region. The formation of self-assembled GaN nanostructures on aluminum nitride (AlN) grown on sapphire substrates by Metal Organic Chemical Vapor deposition (MOCVD) was explored. This paper reports on the effect ofin-situactivation in nitrogen atmosphere on MOCVD grown GaN nanostructures. The effect of introducing manganese in these nanostructures was also studied. Optically active nanostructures were successfully obtained. A blue shift is observed in the photoluminescence data with a decrease in nanostructure size.
Reflectance spectroscopy of single photonic crystal island fabricated by ink-jet printing
Authors: Wang Dake , Park Minseo , Park Jungho , Moon Jooho
Published: April 2011

A micro-reflectance spectroscopy was performed on a single island of photonic crystal array. An array of islands of photonic crystal with colloidal polystyrene beads was assembled on Si using ink-jet printing. The polystyrene colloids with three different sizes (190 nm, 210 nm, and 270 nm in diameter) were used and the polystyrene colloidal particles were self-assembled to form fcc lattices with three different lattice spacing. It was observed from the reflectance spectra that the position of the optical stop band shifts as the size of the colloidal particle changes. Effective medium approximations were used to model the dielectric properties of the colloid/air composite. The theoretically expected reflectance peak position agrees well with those of the experimentally observed peaks. The effect of finite size of the photonic crystal island on its reflectance spectroscopy was investigated by comparing the reflectance spectra collected from four different photonic crystal islands assembled from the same polystyrene colloidal particles, but with different lateral size for each island. It was found that the primary reflection peak was broadened and its intensity was reduced when the lateral size of the island was decreased.
Engineering Three Dimensional Nanotextured Opal-Like Silica Foams
Authors: Carn Florent , Massé Pascal , Ravaine Serge , Backov Rénal
Published: April 2011

Novel meso-/macroporous SiO2 monoliths have been reached by applying a nanotectonic pathway within a confined geometry, i.e. a non-static air-liquid foam patterning process. Final scaffolds are a very close transcription of the tailored periodic air-liquid foam template while coalesced silica particles are texturing the as-synthesized foam walls. The interconnected nanoparticles and associated void space between adjacent particles allow generating intrinsic mesopores, thereby defining hierarchically organized porous scaffolds. The good control over both the air-liquid foam’s water volume fraction and the bubble size allow a rational tuning of the macropore shape (diameter, Plateau border’s width). In contrast with previous study, closed-cell structures can be reached, while the opal like scaffold structure is maintained with thermal treatment, avoiding thus strong shrinkage associated to the sintering effect.
Submicrometer-Sized Polystyrene Particles Packing on Silicon Microfabricated Substrate
Authors: Tanaka Manabu , Shimamoto Naonobu , Tanii Takashi , Ohdomari Iwao , Nishide Hiroyuki
Published: April 2011

The Packing of submicrometer-sized polystyrene particle within micro-boxes; micrometer-sized squarely recessed pits, was achieved using silicon microfabricated substrates and a simple dipping and pulling-up process. A special phenomenon, cubic packing structures of the particles, was observed when using a micro-box substrate with a few times larger side-length than the particle diameters. Several particle packing structures within different sized micro-boxes were demonstrated, and the relationship between the particle packing structures and micro-box sizes were discussed.
Properties of Chemically Modified Carbon Nanotubes
Authors: Kumashiro Ryotaro , Ohashi Hirotaka , Akasaka Takeshi , Maeda Yutaka , Takaishi Shinya , Yamashita Masahiro , Maruyama Shigeo , Izumida Takeshi , Hatakeyama Rikizo , Tanigaki Katsumi
Published: April 2011

Electric transport properties of chemically modificated carbon nanotubes (CNTs) using Si-containing organic molecules were investigated by means of the field effect transistors (FETs) technique. From the results of FET measurements, it was shown that p-type semiconducting CNTs can be converted to n-type ones by exohedral silylation. It is suggested that the electron carrier are doped into CNTs from the additional silyl groups, that is, the electronic properties of CNTs can be controlled by chemically modifications of outer surface.
An atom probe tomography study of grain boundary segregation in nanocrystalline Ni-W
Authors: Detor Andrew , Miller Michael K. , Schuh Christopher A.
Published: April 2011

Atom probe tomography is used to observe the solute distribution in electrodeposited nanocrystalline Ni-W alloys with three different grain sizes (3, 10, and 20 nm) and the results are compared with atomistic computer simulations. The presence of grain boundary segregation is confirmed by detailed analysis of composition fluctuations in both experimental and simulated structures, and its extent quantified by a frequency distribution analysis. In contrast to other nanocrystalline alloys, the present Ni-W alloys exhibit only a subtle amount of solute segregation to the intergranular regions. This finding is consistent with quantitative predictions for these alloys based upon a thermodynamic model of grain boundary segregation.
Dislocation Instability in Nanoscale Particles
Authors: Carlton Christopher , Ferreira Paulo
Published: April 2011

Understanding the mechanical behavior of nanoparticles has become a key factor for the success of many engineering applications. Classically, for polycrystalline materials one would expect an increase in the yield strength for smaller grain sizes according to the empirical Hall-Petch equation. However, at the nanoscale, this mechanism seems to break down. Despite the number of interesting papers published on this topic, controversy persists and consensus is still lacking. While this discussion is of great interest for polycrystalline nanoparticles, contemporary models for strengthening are inappropriate for single-crystal nanoparticles, which, altogether, pose a very different problem: the absence of grain boundaries to inhibit dislocations. In this context, we present a new model wherein the change in Gibbs Free Energy of an edge dislocation in a single-crystal particle provides a driving force for dislocation motion. These results show that dislocations become unstable and are spontaneously ejected from the particles below a certain critical size.
Mechanical Behavior of Multi-Principal-Element Fe0.71Cr1.48Al0.38Ni1.42Co Alloy
Authors: Wang Woei-Ren , Chen Hung-Cheng , Lai Jiun-Hui , Tu Chin-Pang , Sheu Tsing-Shien
Published: April 2011

The conventional concept of alloy design is based on the principal metallic element (>40 at.%); therefore, it is novel and challenging for a new alloy design to synthesize several (5∼7) principal elements (each <35 at.%), i.e. equal or near-equal molar ratio. With the design concept of multi-principal-element alloy, this paper will present the mechanical behaviors and workability of this alloy, Fe0.71Cr1.48Al0.38Ni1.42Co in molar ratio. The alloy possessed the simple FCC crystal structure even though these elements come from three categories of crystalline structures, FCC, BCC and HCP. The hardness can also reach 367(Hv) after precipitated treatment at 700°C for 24 Hr.. Meanwhile, the ultimate tensile strength (UTS) of Fe0.71Cr1.48Al0.38Ni1.42Co can be elevated to 1112 MPa as almost triple as the original sample, and the elongation rate can be kept at 20%.
A Mother-Daughter Mechanism of Mode I cracks: Supersonic Crack Motion Along Interfaces of Dissimilar Materials
Authors: Buehler Markus J. , Gao Huajian
Published: April 2011

In this paper we summarize recent progress in applying large-scale atomistic studies of crack dynamics along interfaces of dissimilar materials. We consider two linear-elastic material strips in which atoms interact with harmonic potentials, with a different spring constant in each layer leading to a soft and a stiff strip. The two strips are bound together with a weak potential whose bonds snap early upon a critical atomic separation. An initial crack serves as initiation point for the failure. We will focus on the maximum speed of mode I loaded cracks along such a biomaterial interface. Upon nucleation of the crack, it quickly approaches a velocity a few percent larger than the Rayleigh-wave speed of the soft material. After a critical time, we observe that a secondary crack is nucleated a few atomic spacings ahead of the crack. This secondary crack propagates at the Rayleigh-wave speed of the stiff material. If the elastic mismatch is sufficiently large, the secondary crack can be faster than the longitudinal wave speed of the soft material, thus propagating supersonically. Supersonic crack motion is clearly identified by two Mach cones in the soft material. This suggests that mother-daughter mechanisms, formerly only reported in mode II cracks in homogeneous materials, play an important role in interfacial mode I crack dynamics. The studies reported in this paper exemplify the usage of large-scale atomistic simulation to investigate the physics of fracture.
Unit Shearing Events in Plasticity of Amorphous Silicon
Authors: Demkowicz Michael J. , Argon Ali S.
Published: April 2011

Plasticity in amorphous silicon (a-Si) as modeled by the Stillinger-Weber (SW) potential was investigated using structure relaxation by potential energy minimization. Irreversible stress drops in the observed mechanical response are the source of plastic deformation in this model directionally bonded material. Every such stress drop was found to be accompanied by atomic rearrangements that give evidence of the existence of unit inelastic shearing events that are characteristic of a-Si and account for plasticity in this material.
Fractal Dimension for Ceramic Fracture Surface
Authors: Bielecki Jerzy , Kotowski Piotr
Published: April 2011

An investigation of an influence of loading parameters on fractal dimension of electrotechnical ceramics fracture has been presented. Investigation objects were samples made of C110 and C130 ceramics. Specimens were breaking under cyclic and static loading conditions. To estimate fractal characteristics a method used for metals fracture had been adapted. Fractal dimensions were estimated for profiles obtained by the vertical cross section method. Obtained results show that fractal dimension does not characterises the material. That dimension does not characterises loading conditions, as well.
Indentation Crystallization and Phase Transformation of Amorphous Germanium
Authors: Patriarche G. , Le Bourhis E. , Khayyat M.M. , Chaudhri M.M.
Published: April 2011

It has been known for about 15 years that when a Vickers indenter is loaded on to a crystalline semiconductor, such as silicon, a semiconductor to metallic phase transition occurs during indenter loading and on removal of the indenter the material within the residual indentation becomes amorphous. Here we report on a completely opposite effect: when a Berkovich or Vickers diamond indenter is loaded on to a submicrometre thick film of amorphous germanium, it densifies, crystallizes and undergoes structural phase transitions. These observations are based on transmission electron microscopy and Raman scattering investigations. It has also been shown that the indentation-induced crystallization and phase transitions occur close to the indenter tip, where the plastic strains are the highest.
Deformation Mechanisms in Refractory Rare-Earth Phosphates
Authors: Hay Randall , Mogilevsky Pavel
Published: April 2011

A method for prediction of deformation twin modes from analysis of the necessary atomic shuffles was developed for monazite (monoclinic LaPO4). This method is applied to scheelite (tetragonal CaWO4), xenotime (tetragonal YPO4), and zircon (tetragonal ZrSiO4). The predictions are compared to observations in these materials. Merit of the predictive method is discussed.
Nanoscale Measurements in Organic Memory Devices from C60 in Insulating Polymers
Authors: Kanwal Alokik , Chhowalla Manish
Published: April 2011

Following our pervious works demonstrating all-organic memory devices based on a nanocomposite material consisting of C60 molecules dispersed in an insulating polymer. In this paper, we will report on conducting atomic force microscopy (c-AFM) measurements from nanosized regions on memory devices. The c-AFM nanoscale measurements show a hysteresis of high and low conductance states, in agreement with our previous reports on macroscopic memory devices. The c-AFM measurements were verified by 30nm gap cell devices fabricated via e-beam lithography, which also showed similar current values. Analysis of our data reveals that the conduction mechanism switches from direct tunneling to Fowler-Nordheim tunneling above a threshold voltage.
Two-Dimensional Carbon Nanotube Networks: A Transparent Electronic Material
Authors: Gruner George
Published: April 2011

The random, two-dimensional network formed of electrically conducting nanoscale wires, called carbon nanotubes, is a transparent electronic material that can be fabricated using room-temperature printing or spraying technologies. Depending on the network density, networks with both metallic- and semiconducting-like attributes can be fabricated. Both display high conductivity, high carrier mobility and optical transparency. The networks also have high mechanical flexibility, robustness and environmental resistance. Application opportunities range from lightweight, transparent conducting films, to electrically conducting fabrics, to active electronic devices and sensors.
Free-standing Diamond Single Crystal Film for Electronics Applications
Authors: Yang Jie , Huang Weixiao , Chow T. P. , Butler James E.
Published: April 2011

High quality single crystal diamond film is an excellent transparent semiconductor material. Combined with its good electrical, optical, thermal and chemical properties, diamond-based semiconductor devices offer the potential of operation at very high voltages (>10 kV), power levels, and temperatures (>400°C) and under extreme radiation conditions.

In this paper, we exploit the optical transparent property of MPCVD single crystal diamond films to correlate the quality of the epi-layers with the performance of Schottky barrier diodes fabricated on the layer. We used optical microscopy to observe stress induced birefringence caused by defects/dislocations in the material and micro- Raman/photoluminescence to detect relative amounts of non-diamond carbon and color centers (nitrogen and silicon atom complexes with lattice vacancies) in the material. High structural quality (low stress) is correlated with the properties of Schottky barrier diodes fabricated in the material. Vertical devices made from a 20 µm homo-epi-layer have been shown high breakdown fields of 1.85 MVcm-1 (BV = 3.7 kV) and conduction of 0.6 A/cm2 at 20V forward drop at 290 °C. Through device failure analysis, we can conclude that the 1.85 MVcm-1 field is only a lower limit for the material. Local stresses (dislocations) and point defects appear to be the main reasons for the high voltage failure of our single crystal diamond rectifiers.
Four-probe Magnetoresistance of Current-perpendicular-to-plane Structures
Authors: Zhou Hua , Covington Mark W. , Seigler Michael A.
Published: April 2011

The resistance and magnetoresistance (MR) of three-dimensional current-perpendicular-to-plane (CPP) structures have been calculated via numerical finite element solutions of the Laplace equation. This model accounts for the non-uniform current paths in a four-probe geometry that can yield MR that differs from the intrinsic MR of the isolated CPP pillar with spatially uniform current flow. We calculated the four-probe MR for various geometries and resistivities of both the normal metal leads and the magnetoresistive pillar. From a single, unified approach, we are able to consistently account for the disparate behavior that has been previously published. In particular, we identify conditions that produce four-probe MR that differs from the intrinsic MR of the CPP pillar and highlight those situations where the four-probe resistance is negative. Finally, we present a simple analytical formula for the MR ratio that is applicable to narrow CPP pillars with wide, thin leads.
Magnetic Field Sensors based on Magnetoresistance Effect in Organic Semiconductor Sandwich Devices
Authors: Veeraraghavan Govindarajan , Mermer Ömer , Sheng Yugang , Nguyen Tho Duc , Francis Thomas Lee , Wohlgenannt Markus
Published: April 2011

We describe magnetic field sensors based on a recently discovered magnetoresistance (MR) effect in nonmagnetic organic semiconductor sandwich devices. The MR effect reaches up to 10% in a magnetic field of 10 mT at room temperature. We perform an extensive experimental characterization of this effect. We found that the MR effect is only weakly temperature dependent and does not depend on sign and direction of the applied magnetic field. We also measured the device response to alternating magnetic fields up to 100 kHz. To the best of our knowledge, the discovered MR effect is not adequately described by any of the MR mechanisms known to date.
Minimizing 1/f Noise in Magnetic Sensors using an AC MEMS Flux Concentrator
Authors: Edelstein Alan , Fischer Greg A. , Pedersen Michael , Nowak Edmund , Cheng Shu Fan , Nordman Cathy A
Published: April 2011

We are developing a device, the MEMS flux concentrator, that will greatly decrease the effect of 1/f noise in magnetic sensors. It does this by modulating the incoming signal and thus shifting the operating frequency of the sensor. This is accomplished by placing flux concentrators on MEMS structures that oscillate at kHz frequencies. Depending upon the sensor, shifting the operating frequency reduces the 1/f noise by one to three orders of magnitude at one Hz. We have succeeded in fabricating the necessary MEMS structures and observing the desired kHz normal mode resonant frequency. Only microwatts are required to drive the motion. We have used spin valves for our magnetic sensors. The measured field enhancement provided by the flux concentrators agrees to within 3% with the value estimated from finite element calculations. Noise measurements provide strong evidence that the device is likely to reduce the effect of 1/f noise. Flip chip bonding is likely to allow us to fabricate complete, fully functioning sensors.
In-situ Observations of Martensitic Transformation in Pure Ti Thin Films using the Dynamical Transmission Electron Microscope (DTEM)
Authors: LaGrange Thomas Bradley , Campbell Geoffrey H. , Colvin Jeffrey D. , King Wayne E. , Browning Nigel D. , Armstrong Michael R. , Reed Bryan W. , Kim Judy S. , Stuart Brent C.
Published: April 2011

We have measured the transient events of the α-β martensitic transformation in nanocrystalline Ti films via single shot electron diffraction patterns with 1.5 ns temporal resolution. This was accomplished with a newly constructed dynamic transmission electron microscope (DTEM), which combines pulsed laser systems and pump-probe techniques with a conventional TEM. The DTEM thereby enables studies of transformations that are (1) far too fast to be captured by conventional bulk techniques, and (2) difficult to study with current ultrafast electron diffraction (UED) instruments (which typically require an accumulation of multiple shots for each diffraction pattern). Martensitic transformations in nanocrystalline materials meet both criteria, with their rapid nucleation, characteristic interface velocities ∼1 km/s, and significant irreversible microstructural changes. Free-standing 40-nm-thick Ti films were laser-heated at a rate of ∼1010K/s to a temperature above the 1155 K transition point, then probed at various time intervals with a 1.5-ns-long intense electron pulse. Diffraction patterns show an almost complete transition to the β phase within 500 ns. Post-mortem analysis (after the sample is allowed to cool) shows a reversion to the α phase coupled with substantial grain growth, lath formation, and texture modification. The cooled material also shows a complete lack of apparent dislocations, suggesting the possible importance of a "massive" short-range diffusion mechanism.
In Situ Recrystallization Imaging of Polycrystalline Ni-S Alloy by SEM
Authors: Cornen Marilyne , Le Gall René
Published: April 2011

Impurity segregation at alloy grain boundaries (GB) depends on many parameters such as GB character and prior thermomechanical history. In this paper we describe an experimental set-up designed to record the individual behaviour of GB during recrystallization of a Nickel-Sulphur alloy. The aim is to correlate segregation level with mobility and disorientation of GB. Therefore a method allowing direct observation of the GB motion as well as disorientation mapping is needed.

Studying grain growth and recrystallization of this particular alloy by Scanning Electron Microscopy is possible thanks to real time in-situ imaging. This requires a sensitive crystal orientation contrast and the ability to image hot evolving samples. To heat the samples, a special device designed for the SEM analysis chamber is used. This device is used at 455°C but is able to reach higher temperatures. In order to get the orientation contrast images, as well as to protect the Secondary Electron (SE) detector from heat and light, a channel plate converter has been designed. This experimental set-up, combined with the heating system, allow the observation of the sample during recrystallization and recording small videos providing information such as grain boundary displacement rate. Then, associated with an EBSD analysis we can get the disorientation map of the area of interest and eventually an electrochemical technique is performed to create preferential dissolution at GB. Geometry of the grooves resulting from an electrochemical etching gives an evaluation of the segregation level in GB.
Pd/Ni(111) surface structure by impact-collision ion scattering spectroscopy
Authors: Umezawa Kenji , Yamauchi Kaname , Nakanishi Shigemitsu , Gibson Walter M
Published: April 2011

Pd deposits on Ni(111) have been studied by low energy ion scattering spectroscopy. We found that two different types of epitaxial growth exist: Pd[11 2]//Ni[11 2] and Pd[11 2]//Ni[11 2] at substrate temperature of 300 K. The Pd(111) planes grow as mixed domains of 50 % each. Surface alloying of the Pd-Ni system was confirmed for deposition at 300 K in which Pd atoms displace first-layer Ni atoms, with an outward displacement of 0.4 Å with respect to first layer Ni atoms.
Ion-Beam-Synthesised Ag-SiO2 Nanocomposite Layers for Electron Field Emission Devices
Authors: Tsang Wei-Mong , Stolojan V. , Adikaari A. A. D. T. , Wong S. P. , Silva S. R. P.
Published: April 2011

Ag-SiO2 nanocomposite layers were synthesised by Ag+ implantation into thermally oxidised SiO2 layers and demonstrated to have excellent field emission (FE) properties. These nanocomposite layers can give an emission current of 1 nA at electric fields less than 20 V/μm, compared to several thousand volts per micrometre of pure metal surfaces. Their fabrication processes are fully compatible with existing integrated circuit technology. By correlating the FE results with other characterisation techniques including atomic force microscopy, Rutherford backscattering spectroscopy and transmission electron microscopy, it is clearly demonstrated that there are two types of field enhancement mechanisms responsible for the excellent FE properties of these cathodes. Firstly, the electrically conductive Ag nano-clusters embedded in the insulating SiO2 matrix give rise to a local electric field enhancement due to an electrical inhomogeneity effect and secondly, the dense surface protrusions provide a geometric local electric field enhancement. The FE properties of these layers are critically dependent on the size and distribution of the Ag clusters, which can be controlled by the Ag dose and modified by the post-implantation pulse annealing with a high power KrF Excimer laser operating at 248 nm.