Bulk Scales Research Articles

Synthesis and Characterization of One-Dimensional Cu(II) Coordination Polymer in Bulk and Nano Scales: New Precursor for Preparation of Cu(II) Oxide Nanoparticles

The hydrothermal reaction of copper(II) acetate and 1,3,5-benzenetricarboxylic acid (btcH3) leads to the formation of a coordination polymer, [Cu(btcH)(H20)3]n (1). The product was characterized by elemental analysis and IR spectroscopy. Nanoparticles of coordination polymer (1) have been synthesized by a sonochemical process and characterized. Powder XRD and IR spectra of the compound 1 both its bulk and nano-size indicate acceptable matches for the structure. Thermal decomposition of the nano-size [Cu(btcH)(H20)3]n as a precursor leads to the formation of CuO nanoparticles. Characterization of the CuO nanoparticles was carried out using IR spectroscopy, powder XRD and scanning electron microscopy. This study demonstrates that the coordination polymers may be suitable precursors for the preparation of nanoscale materials.

Thlaspi arvense binds Cu(ii) as a bis-(l-histidinato) complex on root cell walls in an urban ecosystem

Root cell walls accumulate metal cations both during acquisition from the environment and removal from the protoplast to avoid toxicity, but molecular forms of the metals under field conditions remain elusive. We have identified how copper is bound to cell walls of intact roots of native Thlaspi arvense by combining synchrotron X-ray fluorescence and absorption techniques (XANES and EXAFS) at the nano-, micro-, and bulk scales. The plants grew naturally in sediment in a stormwater runoff basin at copper concentrations typical of urban ecosystems. About 90% of acquired copper is bound in vivo to cell walls as a unique five-coordinate Cu(II)-bis(L-histidinato) complex with one L-histidine behaving as a tridentate ligand (histamine-like chelate) and the other as a bidentate ligand (glycine-like chelate). Tridentate binding of Cu(II) would provide thermodynamic stability to protect cells against copper toxicity, and bidentate binding may enable kinetic lability along the cell wall through protein-protein docking with the non-bonded imidazole group of histidine residues. EXAFS spectra are provided as ESI to facilitate further identification of Cu-histidine and distinction of Cu-N from Cu-O bonds in biomolecules.

Ejecta source and transport modeling in the FLAG hydrocode

We present the ongoing development and implementation of an ejecta model in the FLAG hydrocode. Ejecta is the term given to particulate matter that is produced at the free surface of a material subject to extreme shock loading. Following shock propagation into a material and reflection at its free surface, conditions may be sufficient to induce phase changes, damage, or fragmentation at the surface. The dynamics of the fragmentation may be such that a “cloud” of particulate matter forms and propagates away from the material.Modeling such phenomena in a continuum hydrodynamics code challenges the assumptions underlying the numerical approximations made in the hydrodynamics. The representative scales for the particulate matter are often much smaller than the representative scales for the bulk material producing the ejecta. However, this scale separation allows for statistical descriptions of ejecta that are compatible with continuum mechanics.Earlier work documents an initial effort in modeling ejecta in the FLAG hydrocode. The FLAG hydrocode computes continuum mechanics solutions for fluid and solid materials in an Arbitrary–Eulerian–Lagrangian (ALE) framework. To model ejecta in FLAG, a hybrid particle-continuum representation was defined that allows for coupling with continuum materials on large (bulk) scales. Numerical models were developed and implemented for particle production (sourcing) as well as for solving the particle equations of motion. The numerics were shown to conserve mass, momentum and energy, and preliminary results were given for modeling drag and volume effects.This work documents recent advances in source and transport models. Spatial and temporal dependencies have been added to the source models to account for geometric free-surface variations, mesh dependence, and shock loading. More physically relevant drag models have been implemented that include Reynolds number effects. These will be presented along with test results verifying the models. A FLAG model of an actual ejecta experiment will also be presented.

Micromechanical modeling and analysis of different flow regimes in gas fluidization

The micromechanics of different particle–fluid flow regimes, such as fixed, expanded and fluidized beds, in gas fluidization is investigated for group A and B powders. To establish the connection between macroscopic and microscopic descriptions of complex particle–fluid flows, focus is given to the following two aspects: the formation of a stable expanded bed in relation to the interparticle cohesive, sliding and rolling frictional forces, and the correlation between coordination number (CN) and porosity (ε). The method employed is the combined approach of three-dimensional (3D) discrete element method (DEM) and two-dimensional (2D) computational fluid dynamics. The results show that compared to 2D DEM, 3D DEM is more reliable in investigating the micromechanics of granular media, although both can capture key features of different flow regimes. The roles of various forces between particles and between particles and fluid are examined, and the origin of different flow regimes is discussed. It is shown that the cohesive force is critical to the formation of a static expanded bed, while the sliding and rolling frictional forces also play a role here. The criterion for bed expansion is analyzed at bulk and particle scales, and the deficiency at a bulk scale is identified. CN, as a key measure of local structure, is analyzed. It is found that the CN–ε relationship for group A powders has a transitional point between the expanded and fluidized bed flow regimes at a bulk scale, unlike group B powders. A new phase diagram is established in terms of CN–ε relationship that has two branches representing expanded and fluidized (bed) states, which corresponds to the one in terms of interparticle forces.

Phase and morphological segregation in Ti-MCM-41

In this study Ti-MCM-41 materials have been synthesised and characterised with a view to developing Ti anchoring sites for supporting metal nanoparticles. Up to 20%Ti has been incorporated into the matrix using titanium iso-propoxide. A soft-templating approach has been followed, and the reaction is carried out under highly alkaline conditions. The N 2-sorption and XRD results indicate that a hexagonal mesoporous material is present. However, electron microscopy characterisations indicate that phase and morphological heterogeneities exist. At the atomic scale two phases of oxide particles are formed: a hexagonal mesoporous silicate phase and an amorphous porous titanosilicate phase. At the bulk scale these particles gather into aggregates (>100 μm) which are segregated into Ti-rich and Si-rich entities. Thus, the heterogeneity occurs at both atomic and bulk scales. Although clearly revealed by electron microscopy, these heterogeneities are impervious to detection by the traditional N 2-sorption and low-angle XRD approaches.

Toward a Reliable Synthesis of Strontium Ruthenate: Parameter Control and Property Investigation of Submicrometer-Sized Structures

SrRuO3 (SRO) has been studied extensively at the bulk and thin film scales for a variety of applications, such as multilayer devices, field-effect devices, multiferroics, and even ferroelectric random access memory (FeRAM). Conversely, the exploration of SRO nanoscale and submicrometer scale structures in terms of both synthesis and applications has been significantly limited. Herein, we are the first to report on the synthesis of SRO submicrometer-sized particles via a molten-salt method. We have accomplished this by systematically probing experimental parameters such as precursors, type of salt, annealing time, annealing temperature, surfactant, cooling rate, and reaction atmosphere in an effort to predictably control the resulting size, shape, and morphology of the SRO product particles. In particular, by quenching the reaction at a cooling rate of 100 °C/min, we can produce rounded SRO particles averaging 149 ± 100 nm in size. Moreover, with the addition of a mixture of mineral oil in Triton X-100, the final SRO particles are highly faceted, single-crystalline octahedra, averaging 126 ± 45 nm in size. Apart from the choice of molten salt which primarily controls chemical composition of SRO, we have determined the most important experimental parameter for shape and aggregation control is the surfactant, because of a combination of its hydrophobic and hydrophilic characteristics. We have found that the decomposition of the surfactant promotes the formation of SrCO3, which is consistent with the generation of SRO through the reaction of SrCO3 with RuO2. Our successful syntheses have allowed us to explore physical properties, that is, magnetic and electronic, of these submicrometer-sized SRO particles, which are in good agreement with bulk SRO. Furthermore, we have also explored the potential of our as-synthesized particles as effective methanol oxidation reaction (MOR) catalysts for direct methanol fuel cells (DMFCs). Not only are our as-synthesized SRO particles MOR active but also our well-defined faceted octahedra exhibit a 4-fold increase in mass-activity and a 4-fold increase in surface area by comparison with the rounded particles, thereby emphasizing a clear advantage of faceted submicrometer SRO for electrochemical applications.

Size dependence of microprobe dynamics during gelation of a discotic colloidal clay

Soft materials, such as gels and colloidal glasses, often exhibit different rheological properties at bulk and microscopic scales as a result of their complex microstructure. This phenomenon has recently been demonstrated for a gel-forming aqueous dispersion of Laponite® clay [Oppong et al. Phys. Rev. E 78, 021405 (2008)]. For this material, microrheology reveals a significantly weaker gel and a longer gelation time than bulk measurements. By performing multiple particle tracking microrheology experiments with different probe sizes, we show that length-scale–dependent rheology is a general feature of Laponite® gels. Small changes in probe size are accompanied by order of magnitude differences in the observed rheological properties and gelation time. The probe dynamics also exhibit size-dependent spatial heterogeneities that help to elucidate a microstructural length scale in the system. Through analytical theory and Brownian dynamics simulations, we find that the correlations described by previous authors between successive displacements of individual probes are more directly a result of material elasticity than of microstructural confinement. The apparent gelation times of dispersions with different Laponite® concentrations exhibit a self-similar dependence on probe size, suggesting a superposition of Laponite® concentration and probe size. From these observations, we propose an accordant description of the microstructural evolution of the gel.

Characterization of particle and bulk level cohesion reduction of surface modified fine aluminum powders

In this work, characterization of cohesion reduction at the particle as well as bulk scales is addressed, including the investigation of analytical models to relate the properties at two scales. Two techniques, surface silanization and dry particle coating, are utilized to reduce the cohesiveness and improve the flowability of fine cohesive aluminum powders. Cohesiveness of these surface modified powders is evaluated at the particle scale through the particle properties such as surface roughness and surface energy, and at the bulk scale in both consolidated and aerated state using a Sevilla Powder Tester through unconfined tensile strength, solid fraction, settling and bubbling velocity. In addition, bulk-scale characterization using standard Angle of Repose (AoR) method is carried out. An analytical model is proposed, which allows for calculating interparticle pull-off force where the representative surface morphology is randomly generated and the plastic deformation of asperities in contact is taken into account. Experimental results indicate that surface silanization and dry particle coating can dramatically improve the flowability, and make otherwise unfluidizable powder fluidizable. The reduction of cohesiveness is largely ascribed to the reduction of surface free energy, which becomes less than 1/4 of the original value. Bond number is computed for all samples from the interparticle pull-off force and measured particle size. Surface modification drastically reduces the Bond number by approximately 2 orders of magnitude indicating the significant improvement of flowability. Representation of particle cohesiveness by Bond number has good qualitative agreement with the bulk-scale cohesiveness characterized by the Angle of Repose as well as the bubbling velocity which is measured using the Sevilla Powder Tester.

Multi-scale methods and complex processes: A survey and look ahead

A comprehensive overview of multi-scale methods is presented with a focus on the ways in which information is communicated between scales. Liquid density computations, which are important in phase equilibrium of carbon storage, are used to illustrate multi-scale process engineering ideas. It is shown that using the Gibbs–Helmholtz equation to constrain the energy parameter in cubic equations of state leads to (1) a natural bridge between bulk and molecular length scales, (2) a new Gibbs–Helmholtz constrained equation of state, and (3) a novel mixing rule, and that the GHC equation can be coupled to NTP molecular simulations to provide more accurate predictions of density for use in phase equilibrium computations and reservoir simulations. Improvements for the next generation of multi-scale tools for analysis, visualization, simulation, and optimization of complex processes including the need to proceed with partial knowledge, multiplicity at various length scales, problems with many scales, and methods for predicting behavior over very long time and length scales are also discussed.

A review of microstructure in concentrated suspensions and its implications for rheology and bulk flow

An overview of present understanding of microstructure in flowing suspensions is provided. An emphasis is placed on how the microstructure leads to observable bulk flow phenomena unique to mixtures. The bridge between the particle and bulk scales is provided by the mixture rheology; one focus of the review is on work that addresses the connection between microstructure and rheology. The non-Newtonian rheology of suspensions includes the well-known rate dependences of shear thinning and thickening, which have influence on bulk processing of suspensions. Shear-induced normal stresses are also measured in concentrated suspensions and include normal stress differences, and the isotropic particle pressure. Normal stresses have been associated with shear-induced migration, and thus have influence on the ultimate spatial distribution of solids, as well as the flow rate during processing; a second focus is on these uniquely two-phase behaviors and how they can be described in terms of the bulk rheology. An important bulk fluid mechanical consequence of normal stresses is their role in driving secondary flows.

Fabrication and Nanocompression Testing of Aligned Carbon‐Nanotube–Polymer Nanocomposites

The exceptional electronic, thermal, and mechanical properties of carbon nanotubes (CNTs) have motivated extensive research on their manufacturing and applications. At bulk scales, there is particular interest in property enhancements by adding CNTs to polymers to make composite materials. Most work on CNT-based composites presented in the literature to date has focused on dispersion of single- or multiwalled CNTs (SWNTs or MWNTs) in the matrix; however, bulk CNTs embedded in a polymeric matrix tend to form aggregates that are not only poorly adhered to the matrix but also concentrate stresses, compromising the effect of the CNTs as reinforcement. [1] On the other hand, establishing order and alignment among CNTs within a composite matrix offers significant further potential to harness the properties of individual CNTs at bulk scales by realizing the anisotropic properties of CNTs in desired directions, and by enabling packing and dispersion of CNTs at much higher volume fractions than in tangled configurations. In this work, CNT–polymer composites are manufactured by wetting as-grown arrays of vertically aligned CNTs rapidly and effectively (lack of voids) using off-the-shelf polymers. The wetting process not only preserves the alignment of the CNTs, but also allows the controlled manufacturing of nanocomposite test structures. Direct characterization of the mechanical properties of the nanocomposite structures is also presented in this work. The mechanical-reinforcement results support the feasibility of using these CNTarrays in large-scale hybrid advanced composite architectures reinforced with aligned CNTs. Such composites will also benefit from multifunctional property (e.g., electrical conductivity) enhancements owing to the aligned CNTs.

Elastic strains in silver clusters supported on MgO( [formula omitted

This work reports on the change in the silver–silver interatomic distance d Ag–Ag as a function of the coordination number for silver clusters supported on MgO(1 0 0) surfaces. Interatomic distances were determined by means of X-ray absorption spectroscopy (XAS) at the silver K-edge. For coordination number lower than 7 (below an average thickness equivalent to 1 ML), d Ag–Ag is contracted with respect to the bulk parameter and scales with cluster size. For coordination numbers ranging between 7 and 10 (between 1 and 3 ML), d Ag–Ag increases above the bulk value and shows a tendency to match the MgO(1 0 0) substrate. It then relaxes back to the bulk distance. Coordination numbers found herein successfully compare those derived from a model based on grazing incidence small angle X-ray scattering measurements. At low coverage, the XAS data measured by the total electron yield method exhibits diffraction peaks which are consistent with the presence of structurally coherent islands on the MgO substrate.

Real time study of failure events in polymers

Real time observations of submicrometre scale failure mechanisms of injection moulded multiphase blends are extremely difficult to perform. This has been accomplished using a specially designed tensile fixture and sample preparation techniques for a polycarbonate-polyester blend. Blunt or sharp cracks could be developed in the blend which were dependent on the shape of the polyester domains. Their initiation and propagation mechanisms have been examined at the submicrometre and bulk scales. The failures observed in the thin-sections were similar to failure mechanisms observed in the bulk

Nanomaterials: A Membrane-Based Synthetic Approach

Materials with nanoscopic dimensions not only have potential technological applications in areas such as device technology and drug delivery but also are of fundamental interest in that the properties of a material can change in this regime of transition between the bulk and molecular scales. In this article, a relatively new method for preparing nanomaterials, membrane-based synthesis, is reviewed. This method entails synthesis of the desired material within the pores of a nanoporous membrane. Because the membranes used contain cylindrical pores of uniform diameter, monodisperse nanocylinders of the desired material, whose dimensions can be carefully controlled, are obtained. This "template" method has been used to prepare polymers, metals, semiconductors, and other materials on a nanoscopic scale.

Distribution of radon sources and effects on radon emanation in granitic soil at Ben Lomond, California

The abundance and distribution of radioelements on bulk and microscopic scales were investigated in residual granitic-derived soil at a facility for investigating the movement of radon into structures. In bulk soil samples, Ra concentrations range from 0.6 to 1.3 pCi/g, and variations in Ra, Th, and K appear to be controlled mainly by heterogeneities inherited from the parent granitic rock, which contains abundant dikes and inclusions. U in soil and parent rock is concentrated in primary minerals (mainly zircon and sphene), and in secondary sites that are of greater importance for Rn emanation. The main U-bearing secondary sites are weathered sphene, grain boundary coatings, weathered biotite and plagioclase, as well as dense Fe-rich coatings and a REE-phosphate mineral present in near-vertical fracture zones in saprolite underlying shallow loam. Elevated U in these sites generally correlates with high Ti, Al, Fe, and/or P. Preferential distribution of U and Ra on grain boundaries and porous weathered minerals is reflected in relatively high Rn emanation rates in the soil. Highest emanation occurs between 1.3 and 2.3 m depth, where fine pedogenic phasesgibbsite and amorphous silica and Fe-OOH—are most abundant; it is related to fixation of Ra by these phases, which precipitate close to the surface and accumulate at these depths by illuviation. Separation of Ra from U may occur locally, given remobilization of U-series elements from secondary sites, and large differences between Ra and U sorption capabilities of several phases present in the soil. Concentration of U along permeable fracture zones in saprolite suggests that contribution of soil-gas Rn from depth (> 2 m) could be significant to Rn availability near the surface.

Quantum Liouville theory from topologically massive gravity: 1 + 1 cosmological constant as square of 2 + 1 graviton mass

The connection between two-dimensional quantum gravity (Quantum Liouville theory) on the boundary ∂M of the three-dimensional manifold M and three-dimensional Topologically Massive Gravity (TMG) on the manifold M is considered. The two-dimensional cosmological constant is proportional to the square of the topological mass of the three-dimensional graviton, which establishes a connection between world-sheet and bulk scales. Different physical applications are discussed.