Solid Exchange Research Articles

Angular Dipyridyl Ligands 2,5-Bis(4-pyridyl)-1,3,4-oxadiazole and Its 3-Pyridyl Analogue as Building Blocks for Coordination Architectures: Assemblies, Structural Diversity, and Properties

Abstract The rational design and construction of functional coordination architectures, which rely upon the docile assemblages of organic ligands with metal centers, have attracted continuing interest for many years. The selection of suitable organic ligands as building blocks is a key point for the construction of novel coordination architectures with desired structures and functions. This article will briefly review the coordination-driven assembly of angular dipyridyl-type building blocks involving the oxadiazole spacer, namely 2,5-bis(4-pyridyl)-1,3,4-oxadiazole and its 3-pyridyl-donor analog 2,5-bis(3-pyridyl)-1,3,4-oxadiazole, with transition-metal ions. Diversity of the established structural motifs thus far (only considering the coordination interaction) is outlined, illustrating the broad range of coordination frameworks from discrete molecular architectures to infinite extended one-, two-, and three-dimensional (1-D, 2-D, and 3-D) networks. The specific influencing factors such as anion, solvent, and metal-to-ligand ratio on structural assemblies and the interesting properties of anion exchange, magnetism, fluorescence, and guest inclusion of these crystalline solids will also be discussed.

Effect of the reservoir size on gas adsorption in inhomogeneous porous media

We study the influence of the relative size of the reservoir on the adsorption isotherms of afluid in disordered or inhomogeneous mesoporous solids. We consider both an atomisticmodel of a fluid in a simple, yet structured pore, whose adsorption isotherms are computedby molecular simulation, and a coarse-grained model for adsorption in a disorderedmesoporous material, studied by a density functional approach in a local mean-fieldapproximation. In both cases, the fluid inside the porous solid exchanges matterwith a reservoir of gas that is at the same temperature and chemical potentialand whose relative size can be varied, and the control parameter is the totalnumber of molecules present in the porous sample and in the reservoir. Varyingthe relative sizes of the reservoir and the sample within experimental range maychange the shape of the hysteretic isotherms, leading to a ‘re-entrant’ behaviorcompared to the grand-canonical isotherm when the latter displays a jump in density.We relate these phenomena to the organization of the metastable states thatare accessible for the adsorbed fluid at a given chemical potential or density.

Erratum: Restoring the Density-Gradient Expansion for Exchange in Solids and Surfaces [Phys. Rev. Lett.100, 136406 (2008)

Erratum: Restoring the Density-Gradient Expansion for Exchange in Solids and Surfaces [Phys. Rev. Lett.<b>100</b>, 136406 (2008)

Modeling the competition between solid solution formation and cation exchange on the retardation of aqueous radium in an idealized bentonite column

Clays and clay rocks are considered viable geotechnical barriers in radioactive waste disposal. One reason for this is the propensity for cation exchange reactions in clay minerals to retard the migration of radionuclides. Although another retardation mechanism, namely the incorporation of radionuclides into sulfate or carbonate solid solutions, has been known for a long time, only recently has it been examined systematically. In this work, we investigate the competitive effect of both mechanisms on the transport of radium (Ra) in the near-field of a low- and intermediate level nuclear waste repository. In our idealized geochemical model, numerical simulations show that barium (Ba) and strontium (Sr) needed for Ra sulfate solid solutions also partition to the cation exchange sites of montmorillonite (Mont), which is the major mineral constituent of bentonite that is used for tunnel backfill. At high Mont content, most Ra tends to attach to Mont, while incorporation of Ra in sulfate solid solutions is more important at low Mont content. To explore the effect of the Mont content on the transport of radium, a multi-component reactive transport model was developed and implemented in the scientific software OpenGeoSys-GEM. It was found that a decrease of fixation capacity due to low Mont content is compensated by the formation of solid solutions and that the migration distance of aqueous Ra is similar at different Mont/water ratios.

Comment on “Restoring the Density-Gradient Expansion for Exchange in Solids and Surfaces”

A Comment on the Letter by John P. Perdew et al., Phys. Rev. Lett. 100, 136406 (2008). The authors of the Letter offer a Reply.Received 7 July 2008DOI:https://doi.org/10.1103/PhysRevLett.101.239701©2008 American Physical Society

One dimensional numerical simulation of small scale CFB combustors

In this study, a one-dimensional model which includes volatilization, attrition and combustion of char particles for a circulating fluidized bed (CFB) combustor has been developed. In the modeling, the CFB combustor is analyzed in two regions: bottom zone considering as a bubbling fluidized bed in turbulent fluidization regime and upper zone core-annulus solids flow structure is established. In the bottom zone, a single-phase back-flow cell model is used to represent the solid mixing. Solids exchange, between the bubble phase and emulsion phase is a function of the bubble diameter and varies along the axis of the combustor. In the upper zone, particles move upward in the core and downward in the annulus. Thickness of the annulus varies according to the combustor height. Using the developed simulation program, the effects of operational parameters which are the particle diameter, superficial velocity and air-to-fuel ratio on net solids flux, oxygen and carbon dioxide mole ratios along the bed height and carbon content and bed temperature on the top of the riser are investigated. Simulation results are compared with test results obtained from the 50 kW Gazi University Heat Power Laboratory pilot scale unit and good agreement is observed.

Impact of solids fraction and fluid viscosity on solids flow in rotating cans

Abstract Solids motion in a fluid significantly contributes to particulate heat transfer with that fluid, particularly in the radial direction for axially rotating cans. In this paper, the flow pattern, velocity profile and residence time distribution of solids and the solids’ position exchange frequency between the centre and the wall of cans have been investigated using the positron emission particle tracking (PEPT) technique. The results indicate that the solids flow is complex and varies significantly with the solids fraction and the liquid viscosity, but follows specific patterns. The solid trajectories took a “D” shape in water and golden syrup while a reflected “D” shape in dilute golden syrup. A good exchange of solids from wall to the central region was observed with the dilute golden syrup which had a viscosity of 2 Pa s. The optimum solids flow pattern for a fast and uniform heating of canned foodstuffs can be achieved by controlling the liquid viscosity, solids fraction and the solids/liquid density difference under monitoring using PEPT.

Restoring the Density-Gradient Expansion for Exchange in Solids and Surfaces

Popular modern generalized gradient approximations are biased toward the description of free-atom energies. Restoration of the first-principles gradient expansion for exchange over a wide range of density gradients eliminates this bias. We introduce a revised Perdew-Burke-Ernzerhof generalized gradient approximation that improves equilibrium properties of densely packed solids and their surfaces.

Characterization of Materials for a Hydrogen-Based Economy by Cold Neutron Prompt Gamma-Ray Activation Analysis

AbstractAn instrument for cold neutron prompt gamma-ray activation analysis (PGAA) at the NIST Center for Neutron Research (NCNR) has proven useful for the chemical characterization of hydrogen storage materials and other materials of importance to a hydrogen-based economy. The detection limit for hydrogen is less than 10 mg/kg for most materials. Potential hydrogen storage materials that have been characterized by PGAA include single-wall carbon nanotubes with and without boron doping, porous carbons, lithium magnesium imides, and ternary hydrides of various elements. The capability to allow in situ hydrogenation and characterization of materials is currently under development. PGAA has also been used to characterize materials used in hydrogen fuel cells, including solid proton conductors, polymer membrane, and proton exchange membranes. Future upgrades to the instrument will improve detection limits and functionality of the instrument.

Laboratory tests on self-excitation of concentration fluctuations in slurry pipelines

High concentration solid–liquid mixtures that are conveyed by pipeline have been found to develop large amplitude concentration fluctuations.Aclosedloop laboratory circuit is used to investigate the mechanism of self-excitation. According to linear stability theory these concentration fluctuations originate from an adverse relation between the settling flux and solids concentration. Concentration variations amplify due to solids exchange with a bed layer. The internal structure of the flow is investigated by means of concentration profile measurements. The measurements show that self-excited harmonic perturbations quickly deform into sawtooth shape concentration variations. An explanatory non-linear theory is given.

A arte da catira: negócios e reprodução familiar de sitiantes mineiros

Este artigo analisa os negócios costumeiros de sitiantes de Minas Gerais, as catiras, investigando porque se empenham nessas transações, quais destinos dão aos ganhos que obtém, e como constroem sólidas redes de trocas baseadas em relações de confiança. Assim formam, aos poucos, importantes patrimônios materiais e simbólicos. Esse movimento econômico local, regido por regras próprias e articulado perifericamente aos mercados nacionais, é vital para as suas estratégias de reprodução, para gerar, conservar e ampliar os patrimônios. A lógica peculiar dessas trocas explica vários procedimentos, que geralmente são considerados pouco racionais por técnicos e pesquisadores, mas que fazem parte de muitos cálculos, levados a cabo por anos, para ampliar o pecúlio da família.

Solid Phase Hydrodynamics of Three-Phase Fluidized Bed Reactors -- A Convective/Dispersive Phenomena

The objective of this work was to investigate the relative contribution of the convective and dispersive mixing mechanisms to the overall solid phase mixing mechanism for three-phase fluidized bed reactors. Noninvasive Radioactive Particle Tracking (RPT) data were obtained at various operating conditions, reactor diameters and particle systems. The structural wake model was updated and consists of three sub-phases: the particle wake and downflow-emulsion phase following the convective mixing mechanism and the vortex-emulsion phase following the dispersive mixing mechanism.The particle velocity mean and STD increased with the superficial liquid and gas velocity as well as the reactor diameter for each particle phase. Therefore, the extent of mixing increased under these operating parameters. The particle phase's holdup and a new Mixing Mechanism Indicator (MMI), however, exhibited a much more complex trend. For the larger reactor (Dc = 0.292 m), the convective mixing mechanism dominated and at low superficial liquid velocity the contribution of the convective mixing mechanism increased with superficial gas velocity. This trend, however, was reversed when the superficial liquid velocity increased. For the smaller reactor (Dc = 0.10 m), the random movement of the solid dominated. Therefore, the extent of mixing and the mixing mechanism did not follow the same trend. The CFD and the mixing model have to follow both those hydrodynamic properties.Relations to estimate mean and STD particle velocity distribution as well as particle phase holdup were developed. Solid in the established region mainly followed the convective mixing mechanism. Random movement of the solid was mostly observed at the top of the bed, but it was also present in the established region and at the bed bottom. The volume of each region depended on the operating conditions, reactor diameter and particle system. Furthermore, the particle wake expulsion frequency range was similar to the wake shedding frequency found in literature. It is, thus, possible to assume that wake shedding was mainly responsible for the solid exchange between particle phases.

Preparation of a thermal insulating material using electrophoretic deposition of silica particles

The electrophoretic deposition (EPD) of silica nanoparticles was performed to prepare consolidated thermal insulating material. Optimal conditions were found considering the loading contents, the suspension medium, and the electric field intensity. The density and thermal conductivity were changed by applying a mechanical pressure before drying of the gel. Density ranged from 120 to 500 kg m −3 with an average porosity comprised between 0.78 and 0.95. According to the porosity, the average pores size of compressed materials was between 7 and 20 nm. Such materials exhibit a thermal conductivity as low as 0.020 W m −1 K −1 at room temperature and 1 atm pressure. Thermal conductivity ranged from 0.020 to 0.045 W m −1 K −1. Minimal conductivity exists for a certain distance between particles. Boundary resistance between adjacent particles was obtained as function of the porosity and its change interpreted as ones of conductive paths though air molecules and across direct solid–solid exchange between particles.

Carbon Nanotube Free-Standing Film of Pt/MWNTs as a Bifunctional Component in Hydrogen Proton Exchange Membrane Fuel Cells

AbstractConventional fuel cell architecture on one side of the membrane electrode assembly consists of a carbon backing layer, hydrophobic microporous layer (MPL), and a catalyst layer, which is in contact with the solid proton exchange membrane. Pt nanoparticles are deposited onto multi-walled carbon nanotubes (Pt/MWNTs) and a free-standing film of Pt/MWNTs is fabricated to act as the MPL and the catalyst layer in hydrogen fuel cells. The free-standing film of Pt/MWNTs condenses two functions into one bifunctional layer that simplifies the fuel cell fabrication procedure. Fuel cell polarization performance improves when using the free-standing film of Pt/MWNTs without the MPL resulting in a higher peak performance of 1.2 W/cm2 in comparison with 1.0 W/cm2 when in the presence of a MPL.

A convective/dispersive solid phase mixing model for three-phase fluidized bed reactors: Effect of dimensionless numbers

The objective of this work is to identify the relative importance of dimensionless numbers characterizing the convective/dispersive model and the solid phase mixing. A noninvasive radioactive particle tracking (RPT) technique is used to estimate the model parameters and generate tracer curves at various axial positions for two fluidized bed diameters. The dimensionless number representing the solid dispersion in the particle vortex-phase, 1 / Pe pw – pve , may be neglected for Pe pw – pve > 10 3 (Fick's term of the solid in the vortex-emulsion phase may be neglected); this situation represents most of the experiments. The dimensionless standard-deviation of the velocity distribution of the solid rising within the bubble wake, σ pwd * , should not be neglected, but its variation may be neglected. The dimensionless numbers having the highest effect on solid mixing are the fraction of the solid that rises in the bubble wake ( φ pw ) and the ratio of the solid exchange between particle sub-phases over the mean velocity of the solid that are transported by the bubble wake ( St).

Osmotic Dehydration of Apple Cylinders: III. Continuous Medium Flow Microwave Heating Conditions

Continuous flow osmotic drying permits a better exchange of moisture and solids between the food particle and osmotic solution than the batch process. Osmotic drying has been well studied by several researchers mostly in the batch mode. Microwave heating has been traditionally recognized to provide rapid heating conditions. Its role in the finish drying of food products has also been recognized. In this study, the effects of process temperature, solution concentration on moisture loss (ML), solids gain (SG), and mass transport coefficients (k m and k s ) were evaluated and compared under microwave, assisted osmotic dehydration (MWOD) versus continuous flow osmotic dehydration (CFOD). Apple cylinders (2 cm diameter, 2 cm height) were subjected to continuous flow osmotic solution at different concentrations (30, 40, 50, and 60°Brix sucrose) and temperatures (40, 50, and 60°C). Similar treatments were also given with samples subjected to microwave heating. Results obtained showed that solids gain by the samples was always lower when carried out under microwave heating, while the moisture loss was increased. The greater moisture loss strongly counteracted solids gain in MWOD and thus the overall ratio of ML/SG was higher in MWOD than in CFOD.

Mass Variables and Chemical Potentials in the Gibbs and De Donder Formulations of Chemical Equilibrium

The classical thermodynamic treatment of chemical equilibria in closed systems is founded on theoretical formulations by J.W. Gibbs and Th. De Donder. These two theories represent mathematically equivalent energy variation problems, related to each other through a mass variable transformation analogous to coordinate transformations in mechanics. Associated with the change in mass variables, chemical potentials of reactive substances are defined differently in the two theories, and are subject to different sets of constraints. Traditionally, the two sets of constraints have been merged into one, by assuming that the chemical potential represents the same variable in both theories, an assumption that is formally inconsistent with the difference in chemical potential definitions. Merging of constraints is still possible if chemical potentials remain invariant in value as a manifestation of symmetry under a Gibbs – De Donder mass transformation, but such symmetry has not been investigated. Here we demonstrate chemical potential invariance by reformulating De Donder’s theory using Lagrange multipliers, and combining the resulting constraints on chemical potentials with the classical Gibbs constraints, in a phase equilibration thought experiment. A new form of the Gibbs–Duhem (GD) relation is derived in the De Donder mass system. Comparing this relation to the classical GD equation in Gibbs’ component framework, subject to chemical potential invariance, directly yields the linear operator mapping the De Donder mass variables onto the set of Gibbs component masses. Results are illustrated for a simple ion exchange reaction, in the process solving the longstanding problem of determining solid exchanger activities in ion exchanging mixtures. It is pointed out that, even though the Gibbs and De Donder formulations are mathematically equivalent, the formal structure of De Donder’s theory allows a more explicit and systematic treatment of constraints, and also defines chemical masses in a form more naturally adapted to typical experimental measurements.

Synthesis of Mono‐coordinate Iron(II)‐phen Complex via a Solid State Ligand Exchange Process from Iron(II) Oxalate Dihydrate at Room Temperature under Mechanical Stressing

Mono‐coordinate iron(II)‐1,10‐phenanthroline (phen) complex was obtained in a solid state ligand exchange reaction under mechanical stressing by starting from a mixture of iron(II) oxalate dihydrate (FeC2O4 · 2H2O) and phen, which was not available via a thermal route or from iron(II) chloride tetra hydrate (FeCl2 · 4H2O).

A framework for fast 3D solid model exchange in integrated design environment

Exchanging 3D solid models across engineering applications has become increasingly important to integrated design environments (IDEs). However, transferring models among distributed locations via computer networks usually consumes large amounts of network bandwidth, requires powerful machines, and introduces potentially unacceptable latency. This paper presents a software framework to realize fast solid model exchange in IDEs. In particular, this framework integrates two key components: (a) incremental editing that allows solid models to be edited, modified, and propagated in an incremental form, and (b) progressive streaming that enables solid models to be stored, retrieved, transmitted, and reconstructed in a progressive manner. The benefits of this framework include obtaining real-time interaction with complex geometry, and obviating the burden of transferring large amounts of data over networks.

Influence of Gas–Solid Kinetic Energy Exchange Processes on Gas Effusion from Slitpores in Non-equilibrium Molecular Dynamics Simulations

The boundary-driven type non-equilibrium molecular dynamics (BD-NEMD) simulations have been carried out to clarify the influence of solid flexibility on gas effusion through a slitpore, by using a rigid solid model (R-model) and a flexible solid model (F-model). The LJ potential particles with parameters of argon are chosen for both the effusing gas molecules and the solid atoms. It is found that the R-model combined with a velocity scaling technique provides a reasonable effusion flux, probably a maximum in magnitude, compared with the fluxes calculated from the F-model without a fictitious thermostat (a velocity scaling) for effusing molecules. In the F-model, temperature lowering has been observed at the pore exit, being enhanced with an increase in the strength of springs that unite the solid atoms. This observation indicates that the molecules escaping from the pore exit take the excess kinetic energy for evaporation from solid atoms or surrounding molecules, which results in the low exit temperature, and that the rate of gas–solid kinetic energy exchange decreases with increasing spring strength (i.e. decreasing solid flexibility). It is suggested that the effusion flux is influenced by two factors: the solid flexibility and the molar potential energy in the pore.