Showing papers in "Journal of Materials Research in 2005"

The Effect of Electrolyte Composition on the Fabrication of Self-Organized Titanium Oxide Nanotube Arrays by Anodic Oxidation

Abstract: We report on the fabrication of self-organized titanium oxide nanotube arrays of enhanced surface area prepared by anodic oxidation of a pure titanium sheet in electrolyte solutions containing potassium fluoride (KF) or sodium fluoride (NaF). The effects of electrolyte composition and concentration, solution pH, and the anodic potential on the formation of nanotubes and dimensions of the resulting nanotubes are detailed. Although nanotube arrays of length greater than 500 nm are not possible with hydrofluoric acid containing electrolytes [G.K. Mor, O.K. Varghese, M. Paulose, N. Mukherjee, C.A. Grimes, J. Mater. Res. 18, 2588 (2003)], by adjusting the pH of a KF containing electrolyte to 4.5 using additives such as sulfuric acid, sodium hydroxide, sodium hydrogen sulfate, and/or citric acid, we could increase the length of the nanotube-array to approximately 4.4 μm, an order of magnitude increase in length. The as-prepared nanotubes are composed of amorphous titanium oxide. Independent of the electrolyte composition, crystallization of the nanotubes to anatase phase occurred at temperatures ≥280 °C. Rutile formation occurred at the nanotube-Ti substrate interface at temperatures near 480 °C. It appears geometry constraints imposed by the nanotube walls inhibit anatase to rutile transformation. No disintegration of the nanotube array structure is observed at temperatures as high as 580 °C. The excellent structural and crystal phase stability of these nanotubes make them promising for both low- and high-temperature applications.

Effect of magnetite particle size on adsorption and desorption of arsenite and arsenate

Abstract: Numerous studies have examined arsenic adsorption on varying adsorbents including iron oxides, aluminum hydroxides, alumina, and carbon as a means of arsenic removal in drinking water treatments. The objectives of this study were to evaluate the effect of magnetite particle size on the adsorption and desorption behavior of arsenite and arsenate, and to investigate the competitive adsorption between natural organic matter (NOM) and arsenic. Increases in adsorption maximum capacities for arsenite and arsenate were observed with decreasing magnetite particle size. Arsenic desorption is hysteretic, more so with the smaller nanoparticles. Such desorption hysteresis might result from a higher arsenic affinity for magnetite nanoparticles. In the presence of NOM, substantial decrease in arsenic sorption to magnetite nanoparticles was observed. It would be beneficial to thoroughly investigate adsorption and desorption of arsenic on magnetite nanoparticles for further practical purposes.

Measuring fracture toughness of coatings using focused-ion-beam-machined microbeams

Abstract: Measuring the toughness of brittle coatings has always been a difficult task. Coatings are often too thin to easily prepare a freestanding sample of a defined geometry to use standard toughness measuring techniques. Using standard indentation techniques gives results influenced by the effect of the substrate. A new technique for measuring the toughness of coatings is described here. A precracked micro-beam was produced using focused ion beam (FIB) machining, then imaged and loaded to fracture using a nanoindenter.

Plane-strain Bulge Test for Thin Films

Abstract: The plane-strain bulge test is a powerful new technique for measuring the mechanical properties of thin films. In this technique, the stress–strain curve of a thin film is determined from the pressure-deflection behavior of a long rectangular membrane made of the film of interest. For a thin membrane in a state of plane strain, film stress and stain are distributed uniformly across the membrane width, and simple analytical formulae for stress and strain can be established. This makes the plane-strain bulge test ideal for studying the mechanical behavior of thin films in both the elastic and plastic regimes. Finite element analysis confirms that the plane-strain condition holds for rectangular membranes with aspect ratios greater than 4 and that the simple formulae are highly accurate for materials with strain-hardening exponents ranging from 0 to 0.5. The residual stress in the film mainly affects the elastic deflection of the membrane and changes the initial point of yield in the plane-strain stress–strain curve, but has little or no effect on further plastic deformation. The effect of the residual stress can be eliminated by converting the plane-strain curve into the equivalent uniaxial stress–strain relationship using effective stress and strain. As an example, the technique was applied to an electroplated Cu film. Si micromachining was used to fabricate freestanding Cu membranes. Typical experimental results for the Cu film are presented. The data analysis is in good agreement with finite element calculations.

Quantitative evaluation of indentation-induced densification in glass

Abstract: To estimate the ratio of densification to Vickers indentation volume, three-dimensional images of Vickers indentations on several glasses, including silicate glasses and bulk metallic glass (BMG), were obtained before and after annealing using an atomic force microscope. Large volume recovery of Vickers indentation by annealing was observed for all glasses but BMG. Following previous studies, this recovered volume almost corresponded to the densified volume under a Vickers indenter, and the compositional dependence of densification was discussed. The ratios of densification to the total indentation volume for silica and soda-lime glasses were 92% and 61%, respectively. It was concluded that densification was a general property for silicate glasses and that the ratios of densification to the total indentation volume for all the glasses correlated well with Poisson’s ratios of the glasses.

Nanoindentation of polydimethylsiloxane elastomers: Effect of crosslinking, work of adhesion, and fluid environment on elastic modulus

Abstract: With the potential to map mechanical properties of heterogeneous materials on a micrometer scale, there is growing interest in nanoindentation as a materials characterization technique. However, nanoindentation has been developed primarily for characterization of hard, elasto-plastic materials, and the technique has not been validated for very soft materials with moduli less than 5 MPa. The current study attempted to use nanoindentation to characterize the elastic moduli of soft, elastomeric polydimethylsiloxane (PDMS) samples (with different degrees of crosslinking) and determine the effects of adhesion on these measurements using adhesion contact mechanics models. Results indicate that nanoindentation was able to differentiate between elastic moduli on the order of hundreds of kilo-Pascals. Moreover, calculations using the classical Hertz contact model for dry and aqueous environment gave higher elastic modulus values when compared to those obtained from unconfined compression testing. These data seem to suggest that consideration of the adhesion energy at the tip-sample interface is a significantly important parameter and needs to be taken into account for consistent elastic modulus determination of soft materials by nanoindentation.

Spherical indentation creep following ramp loading

Abstract: Elastic-viscoelastic correspondence, utilizing Boltzmann integral operators, was used to generate displacement–time solutions for spherical indentation testing of viscoelastic materials. Solutions were found for creep following loading at a constant loading rate and compared with step-loading solutions. Experimental creep tests were performed with different loading rate–peak load level combinations on glassy and rubbery polymeric materials. The experimental data were fit to the spherical indentation ramp–creep solutions to obtain values of shear modulus and time-constants; good agreement was found between the experimental results and known modulus values. A multiple ramp-and-hold protocol was examined for the measurement of creep responses at several loads (and depths) within the same test. Emphasis is given to the use of multiple experiments (or multiple levels within a single experiment) to test a priori assumptions made in the correspondence solutions regarding linear viscoelastic material behavior and the creep function.

Organic and Nano-Structured Composite Photovoltaics: An Overview

Abstract: Organic photovoltaic devices are poised to fill the low-cost, low power niche in the solar cell market Recently measured efficiencies of solid-state organic cells are nudging 5% while Gratzel’s more established dye-sensitized solar cell technology is more than double this A fundamental understanding of the excitonic nature of organic materials is an essential backbone for device engineering Bound electron-hole pairs,“excitons,” are formed in organic semiconductors on photo-absorption In the organic solar cell, the exciton must diffuse to the donor-accepter interface for simultaneous charge generation and separation This interface is critical as the concentration of charge carriers is high and recombination here is higher than in the bulk Nanostructured engineering of the interface has been utilized to maximize organic materials properties, namely to compensate the poor exciton diffusion lengths and lower mobilities Excitonic solar cells have different limitations on their open-circuit photo-voltages due to these high interfacial charge carrier concentrations, and their behavior cannot be interpreted as if they were conventional solar cells This article briefly reviews some of the differences between excitonic organic solar cells and conventional inorganic solar cells and highlights some of the technical strategies used in this rapidly progressing field, whose ultimate aim is for organic solar cells to be a commercial reality

Switchable window based on electrochromic polymers

Abstract: Syntheses of a new blue EC monomer (ProDOT-MePro), and a new red EC monomer (ProDOP-Et2) are described. Two additional new types of EC monomers based on 3,4-alkylenedioxythiophene include fluorinated EC monomers and an EC monomer including silicon. EC polymer devices having more than one different color EC polymer to enable additional colors to be provided using subtractive color mixing are also described, as well as EC polymer devices incorporating a logo, image, or text, are generally obscured when the device is colored, but become visible when the device is not colored. Also described are EC polymer devices that include a cathodic EC polymer layer, a gel electrolyte, a counter electrode, and a reference electrode. Working prototypes of such devices exhibit significant increases in the speed of transition of the EC device from a colored state to a transparent state.

Compliant thin film patterns of stiff materials as platforms for stretchable electronics

Abstract: A thin film of a stiff material, patterned as a serpentine on a flat elastomeric substrate, can elongate substantially when the substrate is pulled We showed that the film elongates by twisting out of plane, accommodated by the compliance of the substrate and the pattern of the film Consequently, large elongations of the substrate induce small strains in the film, even when the width of the film is much larger than its thickness Such a wide serpentine, or other compliant patterns of stiff materials, can serve as a platform on which electronic circuits can be fabricated This architecture will make electronics elastically stretchable

Nanoscale Morphology and Indentation of Individual Nacre Tablets from the Gastropod Mollusc Trochus Niloticus

Abstract: The inner nacreous layer of gastropod mollusc Trochus niloticus is composed of ∼95 wt% planar arrays of polygonal aragonite-based tablets (∼8 μm wide, ∼0.9 μm thick, stacked ∼40 nm apart) and ∼5 wt% biomacromolecules. High-resolution tapping mode atomic force microscope images enabled nanoscale resolution of fractured tablet cross-sections, the organic component, and deformation of individual nanoasperities on top of tablet surfaces. Nanoindentation was performed on individual nacre tablets and the elastic modulus E and yield stress σy were reduced from elastic-plastic finite element simulations yielding E = 92 GPa, σy = 11 GPa (freshly cleaved samples) and E = 79 GPa, σy = 9 GPa (artificial seawater soaked samples). Images of the indents revealed extensive plastic deformation with a clear residual indent and surrounding pileup.

Understanding the glass-forming ability of Cu50Zr50 alloys in terms of a metastable eutectic

Abstract: Interest in finding binary alloys that can form bulk metallic glasses has stimulated recent work on the Cu-Zr system, which is known to show glass formation over a wide composition range. This work focuses on copper mold casting of Cu50Zr50 (at.%), and it is shown that fully amorphous rods up to 2-mm diameter can be obtained. The primary intermetallic phase competing with glass formation on cooling is identified, and the glass-forming ability is interpreted in terms of a metastable eutectic involving this phase. Minor additions of aluminum increase the glass-forming ability: with addition of 4 at.% Al to Cu50Zr50, rods of at least 5-mm diameter can be cast fully amorphous. The improvement of glass-forming ability is related to suppression of the primary intermetallic phase.

Platelet interlocks are the key to toughness and strength in nacre

Abstract: Nacre, the inner layer of mollusk shells is a composite made of platelets of mineral aragonitic calcium carbonate with a few weight percent organic material sandwiched in between. The organic and nanostructural nuances are often suggested to be the reason for the extreme toughness of nacre. Here we report the presence of interlocks between platelets of nacre from red abalone. We also report and show, using three-dimensional finite element modeling, that interlocks are the key mechanism for the high toughness and strength of nacre. The observed rotation between platelet layers, which were earlier reported as defects of structure, are necessary for the formation of interlocks.

Flexible, conjugated polymer-fullerene-based bulk-heterojunction solar cells : Basics, encapsulation, and integration

Abstract: Organic solar cells based on conjugated polymer:fullerene blends show nowadays efficiencies above 4%. After briefly presenting the science of bulk-heterojunction solar cells, we report herein a shelf lifetime study performed by encapsulating the cells in a flexible and transparent gas barrier material. This method allows lifetimes as reported for glass encapsulation. Moreover, we propose a new approach to pattern organic solar cells and design large-scale modules. This technique, based on selective Nd:yttrium aluminum garnet (YAG) laser etching, potentially enables low-cost, high-speed roll-to-roll operation.

Representative strain of indentation analysis

Abstract: Indentation analysis based on the concept of representative strain offers an effective way of obtaining mechanical properties, especially work-hardening behavior of metals, from reverse analysis of indentation load–displacement data, and does not require measuring of the projected contact area. The definition of representative strain adopted in previous studies [e.g., Dao et al., Acta Mater. 49, 3899 (2001)] has a weak physical basis, and it works only for a limited range in some sense of engineering materials. A new indentation stress-state based formulation of representation is proposed in this study, which is defined as the plastic strain during equi-biaxial loading. Extensive numerical analysis based on the finite element method has shown that with the new formulation of representative strain and representative stress, the critical normalized relationship between load and material parameters is essentially independent of the work-hardening exponent for all engineering materials, and the results also hold for three distinct indenter angles. The new technique is used for four materials with mechanical properties outside the applicable regime of previous studies, and the reverse analysis has validated the present analysis. The new formulation based on indentation stress-state based definition of representative strain has the potential to quickly and effectively measure the mechanical properties of essentially all engineering materials as long as their constitutive behavior can be approximated into a power-law form.

Mg-Ca-Zn bulk metallic glasses with high strength and significant ductility

Abstract: The development of Mg–Ca–Zn metallic glasses with improved bulk glass forming ability, high strength, and significant ductility is reported. A typical size of at least 3–4 mm amorphous samples can be prepared using conventional casting techniques. By varying the composition, the mass density of these light metal based bulk amorphous alloys ranges from 2.0 to 3.0 g/cm3. The typical measured microhardness is 2.16 GPa, corresponding to a fracture strength of about 700 MPa and specific strength of around 250–300 MPa cm3/g. Unlike other Mg- or Ca-based metallic glasses, the present Mg–Ca–Zn amorphous alloys show significant ductility.

Epitaxial Ti2GeC, Ti3GeC2, and Ti4GeC3 MAX-phase thin films grown by magnetron sputtering

Abstract: We have grown single-crystal thin films of Ti2GeC and Ti3GeC2 and a new phase Ti4GeC3, as well as two new intergrown MAX-structures, Ti5Ge2C3 and Ti7Ge2C5. Epitaxial films were grown on Al2O3(0001) ...

Oxidation behavior of a pressureless sintered ZrB2–MoSi2 ceramic composite

Abstract: Ultra-refractory ceramic composites of composition ZrB2 + (5 to 20) vol% MoSi2 were produced by pressureless sintering at 1830 °C under argon atmosphere. Sintering cycles and microstructural analysis point out that at least 20 vol% molybdenum disilicide is necessary for obtaining a dense material. Thereafter, the composite 80 vol% ZrB2 + 20 vol% MoSi2 was used to test the thermal stability under oxidizing environment. Oxidation tests were carried out in flowing synthetic air in a thermogravimetric analyzer from 700 to 1400 °C with exposure time of 30 h. In the low-temperature range (700–1000 °C), the oxidation of the composite resembles that of monolithic ZrB2 ceramics, for temperatures >1200 °C the silica resulting from oxidation of molybdenum disilicide seals the sample surface, preventing zirconium diboride from fast degradation.

Effects of Elevated Temperature Annealing on the Structure and Hardness of Copper/niobium Nanolayered Films

Abstract: We investigated the effects of elevated temperature vacuum annealing on the morphological stability and hardness of self-supported, textured, polycrystalline Cu–Nb nanolayered films with individual layer thickness varying from 15 to 75 nm. Films with layer thickness greater than approximately 35 nm are found to resist layer pinch-off and spheroidization even after long annealing times at 700 °C, while films with layer thickness ∼15 nm exhibit layer pinch-off and evolve into an equiaxed grain microstructure. Nanoindentation measurements indicate almost no change in hardness after annealing for films that retain the layered morphology, in spite of the increase of in-plane grain dimensions. Significant decreases in hardness are noted for films that develop a coarsened equiaxed grain microstructure after annealing. The mechanism that leads to the development of a thermally stable nanolayered structure is analyzed. Also, the relative effects of in-plane grain size and layer thickness on the multilayer hardness are discussed.

Phase stability, phase transformations, and elastic properties of Cu6Sn5:Ab initio calculations and experimental results

Abstract: Among many Sn-based intermetallics, Cu6Sn5 (η and η′) is ubiquitous in modern solder interconnects. Using the published structural models of η and η′ and also related structures, the total energies and equilibrium cohesive properties are calculated from first-principles employing electronic density-functional theory, ultrasoft pseudopotentials, and both the local density approximation (LDA) and the generalized gradient approximation (GGA) for the exchange-correlation energy. The accuracy of our calculations is assessed through comparisons between theoretical results and experimental measurements for lattice parameters, elastic properties, and formation and transformation energies. The ambient-temperature experimental lattice constants of η and η′ are found to lie between the LDA and GGA level calculated zero-temperature lattice constants. The Wyckoff positions in the structural models of η and η′ agree very well with the ab initio results. The calculated formation energy of η′ lies between −3.2 and −4.0 kJ/mol, which is more positive by about 3 to 4 kJ/mol compared to reported experimental data obtained by solution calorimetry. Our systematic differential scanning calorimetry (DSC) experiments show that the η′ → η transformation enthalpy is 438 ± 18 J/mol, which is about 66% higher than the literature value. In view of our DSC results on heating and cooling, the nature of η′ → η and η → η′ is discussed. Our experimental bulk modulus of η and η′, and the heat of η′ → η transformation agree very well with the ab initio total energy calculations at the GGA level. Based on these results, we conclude that other isotropic elastic moduli (Young’s modulus, shear, and Poissons ratio) of η and η′ phases measured by pulse-echo technique are representative of their actual properties. The scatter in experimental elastic constants in the literature may be attributed to various factors, such as the measurement technique (pulse-echo versus nanoindentation), type of specimen (bulk, Cu6Sn5-layer in diffusion couple, thin-film), and anisotropy effects (particularly in Cu6Sn5-layer in diffusion couples).

Relationships Between Initial Unloading Slope, Contact Depth, and Mechanical Properties for Conical Indentation in Linear Viscoelastic Solids

Abstract: Using analytical and finite element modeling, we examine the relationships between initial unloading slope, contact depth, and mechanical properties for spherical indentation in viscoelastic solids with either displacement or load as the independent variable. We then investigate whether the Oliver-Pharr method for determining the contact depth and contact radius, originally proposed for indentation in elastic and elastic-plastic solids, is applicable to spherical indentation in viscoelastic solids. Finally, the analytical and numerical results are used to answer questions raised in recent literature about measuring viscoelastic properties from instrumented spherical indentation experiments.

An environmental transmission electron microscope for in situ synthesis and characterization of nanomaterials

Abstract: The world of nanomaterials has become the real world for most applications in the area of nanotechnology. As postsynthesis handling of materials at the nanoscale level is impractical, nanomaterials must be synthesized directly as part of a device or circuit. The demands of nanotechnology have led to modifications in the design of transmission electron microscopes (TEMs) that enable in situ synthesis and characterization simultaneously. The environmental TEM (ETEM) is one such modified instrument that has often been used to follow gas–solid and/or liquid–solid interactions at elevated temperatures. Although the history and development of the ETEM, also called the controlled atmosphere or environmental cell TEM, is as old as transmission electron microscopy itself, developments in the design of medium-voltage TEMs have succeeded in bringing resolutions down to the subnanometer level. A modern ETEM equipped with a field-emission gun, energy filter or electron energy-loss spectrometer, scanning transmission electron microscopy coils, and bright-field and dark-field detectors can be a versatile tool for understanding chemical processes at the nanometer level. This article reviews the design and operations of a dedicated ETEM. Its applications range from the in situ characterization of reaction steps, such as oxidation-reduction and hydroxylation, to the in situ synthesis of nanomaterials, such as quantum dots and carbon nanotubes. Some examples of the current and the future applications for the synthesis and characterization of nanomaterials are also discussed.

Effect of Fe Segregation on the Migration of a Non-Symmetric Σ5 Tilt Grain Boundary in Al

Abstract: We present an analysis, based upon atomistic simulation data, of the effect of Fe impurities on grain boundary migration in Al. The first step is the development of a new interatomic potential for Fe in Al. This potential provides an accurate description of Al–Fe liquid diffraction data and the bulk diffusivity of Fe in Al. We use this potential to determine the physical parameters in the Cahn–Lucke–Stuwe (CLS) model for the effect of impurities on grain boundary mobility. These include the heat of segregation of Fe to grain boundaries in Al and the diffusivity of Fe in Al. Using the simulation-parameterized CLS model, we predict the grain boundary mobility in Al in the presence of Fe as a function of temperature and Fe concentration. The order of magnitude and the trends in the mobility from the simulations are in agreement with existing experimental results.

Absence of one-to-one correspondence between elastoplastic properties and sharp-indentation load penetration data

Abstract: The connection between parameters that can be measured by means of instrumented indentation with the real mechanical properties has been a matter of discussion for several years. In fact, even hardness is not a readily measurable magnitude since the real contact area depends on both the elastic and plastic properties of the sample. Recently, Dao et al. [ Acta Mater49, 3899 (2001)] proposed a method based on numerical fittings to calculate by a forward-reverse algorithm the elastoplastic properties of a sample from the load-penetration curve obtained with a sharp indenter. This work will show, in contrast, that it is not possible to measure uniquely these mechanical properties of a sample in that way.

Effect of sample size on the solidification temperature and microstructure of SnAgCu near eutectic alloys

Abstract: The degree of undercooling of Sn in near eutectic, SnAgCu solder balls upon cooling at a rate of 1 °C/s from the melt was examined and found to increase linearly with inverse nominal sample diameter (for balls of radius between 100 and 1000 μm). The mean undercooling for SnAgCu solder balls in a flip chip assembly was 62 °C. The microstructures of these different samples were examined by means of scanning electron microscopy. The Sn dendrite arm width was observed to monotonically increase with ball diameter, indicating a possible dependence of the mechanical response of such solder balls upon size.

Preparation of stable magnetorheological fluids based on extremely bimodal iron–magnetite suspensions

Abstract: The high magneto-viscous response of magnetorheological fluids (MRFs) comes from the large size (≈1 μm) of the magnetic particles dispersed in the carrier liquid. Unfortunately, in the absence of a magnetic field, this large size constitutes the origin of some problems facing the technological applications of MRFs. These problems are (i) the instability of the suspensions caused by the fast settling of the high density magnetic particles used, and (ii) the poor redispersibility due to an irreversible aggregation. In this work, we used an electromagnetic induction method to study the stability of MRFs containing micron-sized iron particles dispersed in ferrofluids composed by oleate-covered magnetite nanoparticles dispersed in kerosene. Interestingly, we demonstrated that the sedimentation rate in iron/ferrofluid suspensions can be significantly lower than in iron/kerosene MRFs.

Early interaction between Fe-Cr alloy metallic interconnect and Sr-doped LaMnO3 cathodes of solid oxide fuel cells

Abstract: The initial stages of the interaction between the Fe–Cr alloy metallic interconnect and Sr-doped LaMnO3 (LSM) electrode of solid oxide fuel cells (SOFC) were investigated under cathodic polarization at the temperature range of 700–900 °C. Cr deposits on the Y2O3–ZrO2 (YSZ) electrolyte surface increased with the polarization time. However, it was observed that at the early stages of the reaction, there is no preferential Cr deposition at the three-phase boundary areas at the LSM electrode/YSZ electrolyte interface region. With the decrease of the temperature the Cr deposition reduced significantly, probably due to the significant reduction in the partial pressure of the gaseous Cr species and the cationic diffusivities in the LSM electrode. The results clearly demonstrated that the deposition of Cr species at the LSM electrode/YSZ electrolyte is basically a chemical reaction and kinetically controlled by the nucleation reaction between the gaseous Cr species and the Mn2+ species generated under cathodic polarization.

In situ tensile testing of nanoscale freestanding thin films inside a transmission electron microscope

Abstract: The unique capability of rendering opaque specimens transparent with atomic resolution makes transmission electron microscopy (TEM) an indispensable tool for microstructural and crystallographic analysis of materials. Conventional TEM specimens are placed on grids about 3 mm in diameter and 10–100 µm thick. Such stringent size restriction has precluded mechanical testing inside the TEM chamber. So far, in situ testing of nanoscale thin foils has been mostly qualitative. Micro-electro-mechanical systems (MEMS) offer an unprecedented level of miniaturization to realize sensors and actuators that can add TEM visualization to nano-mechanical characterization. We present a MEMS-based uniaxial tensile experiment setup that integrates nanoscale freestanding specimens with force and displacement sensors, which can be accommodated by a conventional TEM straining stage. In situ TEM testing on 100-nm-thick freestanding aluminum specimens (with simultaneous stress measurement) show limited dislocation activity in the grain interior and consequent brittle mode of fracture. Plasticity at this size scale is contributed by grain boundary dislocations and partial dislocations.

Study of C60 transport in porous media and the effect of sorbed C60 on naphthalene transport

Abstract: In this study, the transport of water-stable “nano-C60 particles” (a term used to refer to underivatized C60 crystalline nanoparticles, stable in water for months) through a soil column (packed with Lula soil, 0.27% organic carbon) was investigated for the first time. Nano-C60 particle breakthrough experiments were conducted at different flow rates, while other column operating parameters remained fixed through all the experiments. Nano-C60 particles were observed to be more mobile at higher flow velocity: at the flow velocity of 0.38 m/d, the maximum percent of nano-C60 breakthrough (C/C0) was 47%; at the flow velocity of 3.8 m/d, the plateau value of nano-C60 breakthrough was 60%; and at the flow velocity of 11.4 m/d, the plateau value of nano-C60 breakthrough was almost 80%. At the low flow velocity (0.38 m/d), which is typical of groundwater flow, nano-C60 particles showed very limited mobility: after about 57 pore volumes, they deposited to the soil column so rapidly that virtually no nano-C60 was detected in the effluent. This observed “favorable deposition” (attachment efficiency α = 1) was probably due to “filter ripening.” Also the release of nano-C60 particles after flow interruption was observed. The transport of naphthalene through the same soil column containing 0.18% nano-C60 particles deposited was measured. A retardation factor of about 13 was observed, possibly suggesting that sorbed nano-C60 particles in the soil column sorbed naphthalene similar to soil organic carbon. An asymmetric naphthalene breakthrough curve was observed, which is possibly due to “sorption nonequilibrium.”

Doubling the critical size for bulk metallic glass formation in the Mg-Cu-Y ternary system

Abstract: Mg-Cu-Y alloys with optimal glass forming ability have been found at off-eutectic compositions. The critical size for bulk metallic glass formation at the pinpointed compositions more than doubles that of the previously discovered eutectic Mg65Cu25Y10 alloy, leading to fully glassy rods with near-centimeter diameters in the ternary system upon copper mold casting. The result is a striking demonstration of the strong composition dependence of the glass forming ability, as well as of the need to scrutinize off-eutectic compositions. The implications of the discovery are discussed.