Terms Of Gas Transport Research Articles

The Interdependence of Macro‐ and Microstructure on Internal‐Reforming Performance in Ni‐YSZ SOFC Anode Supports

AbstractThis paper presents an analysis of the gas‐transport and methane internal‐reforming characteristics of two commercially developed solid‐oxide fuel cell (SOFC) anode supports. The anode supports are fabricated by CoorsTek, Inc. (Golden, CO, USA) and Risø‐DTU (Lyngby, Denmark). While both supports are ceramic‐metallic composites of yttria‐stabilized zirconia and nickel (Ni‐YSZ), their morphological structures and thicknesses are quite different. The CoorsTek support is thick and displays an open microstructure, while the Risø‐DTU support is fairly thin with a tighter morphology. These micro‐ and macrostructural differences lead to significant variations in gas transport and methane internal‐reforming chemistry within the porous support structures that directly affect cell performance. In this study, anode‐support performance is analyzed using the separated anode experiment, a unique tool that decouples anode‐support thermal‐chemistry processes from electrochemical processes typically underway during SOFC operation. Experimental results are interpreted using a detailed computational model. Gas transport is higher in the CoorsTek support despite being nearly five times thicker than the Risø‐DTU support. The methane internal‐reforming performance of the Risø‐DTU anode, however, speaks to its tighter microstructure and resulting higher catalytic surface area. These results highlight the dependence of support performance on macro‐ and microstructure in terms of gas transport and internal‐reforming chemistry.

Local chain mobility dependence on molecular structure in polyimides with bulky side groups: Correlation with gas separation properties

The effect of bulky side groups on the molecular mobility and chain packing of three polyimides derived from dianhydrides with m-terphenyl moieties and 2,4,6-trimethyl-m-phenylenediamine has been studied. Moreover, structure–properties relationships in terms of gas transport have been established. One of the polyimides has no side groups, while the other two have a t-butyl group in 5′ (meta) position on the central ring of the m-terphenyl moiety and one of them has also a pivaloylimino group placed in the 2′ (ortho) position. The changes in chain packing of the membranes were evidenced by wide angle X-ray diffraction (WAXS), whilst the molecular mobility was evaluated from the dielectric sub-ambient secondary relaxation, γ relaxation. The WAXS pattern of the polyimide containing both t-butyl and pivaloylimino side groups evidenced a greater degree of packing regularity than in the other two polyimides. Moreover, the additional pivaloylimino group strongly restricted the local molecular mobility associated with the phenylene-imide ring flips. On the other hand, the gas permeability of the polyimides strongly increased with the introduction of side groups. This increase was mainly related to the increase in the gas diffusivity, which could not be explained by an increase in fractional free volume (FFV). The results presented in this work indicate that the difference in gas diffusivity of the substituted polyimides is consistent with a difference in size distribution of free volume elements.

The magnetic environment in the central region of nearby galaxies

The central regions of galaxies harbor some of the most extreme physical phenomena, including dense stellar clusters, non-circular motions of molecular clouds and strong and pervasive magnetic field structures. In particular, radio observations have shown that the central few hundred parsecs of our Galaxy has a striking magnetic field configuration. It is not yet clear whether these magnetic structures are unique to our Milky Way or a common feature of all similar galaxies. Therefore, we report on (a) a new radio polarimetric survey of the central 200 pc of the Galaxy to better characterize the magnetic field structure and (b) a search for large-scale and organized magnetized structure in the nuclear regions of nearby galaxies using data from the Very Large Array (VLA) archive. The high angular resolution (1–5″) of the VLA allows us to study the central 1 kpc of the nearest galaxies to search for magnetized nuclear features similar to what is detected in our own Galactic center. Such magnetic features play a important role in the nuclear regions of galaxies in terms of gas transport and the physical conditions of the interstellar medium in this unusual region of galaxies.

Gas transport properties of sulfonated copolyimides: influence of structural parameters and relative humidity

Structure-property relationships in terms of gas transport have been established for a large series of sulfonated copolyimides. At anhydrous state, a large range of permeability values can be obtained varying the nature of the non-sulfonated diamine, the sulfonated diamine content and the sulfonated block length. The diffusion coefficient is in all cases the dominant factor that explains the permeability level. The gas permeability coefficients decrease when the copolyimides are hydrated. A particular behaviour has been evidenced on some membranes for carbon dioxide.

Gas transport mechanism in sulfonated polyimides: Consequences on gas selectivity

Structure–properties relationships in terms of gas transport have been established for a large series of sulfonated copolyimides synthesized with a naphthalenic dianhydride, a sulfonated diamine [SD] and various non-sulfonated diamines. For these two phases systems, a phase inversion has been evidenced for sulfonated block content ranging between 30 and 50%. The contribution of the dispersed sulfonated phase to the gas transport greatly depends on the structure of the non-sulfonated diamine. As a consequence, a large range of permeability values can be obtained varying the nature of the non-sulfonated diamine, the sulfonated diamine content and the sulfonated block length. The diffusion coefficient is in all cases the dominant factor in the transport phenomena and good compromise between O 2/N 2 selectivity and O 2 flux can be achieved with these systems.

Sulfonated copolyimides: influence of structural parameters on gas separation properties

Structure-properties relationships in terms of gas transport have been established for a large series of sulfonated copolyimides. The permeability and diffusion coefficients of different gases (H 2, O 2, CO 2, N 2) differing by their size and interaction capacity towards the polymers have been determined. CO 2 sorption experiments have also been carried out allowing a direct determination of the solubility coefficients. The analysis of the transport parameters as a function of the copolyimides structure and composition underlines the predominant role of the non-sulfonated sequences and shows that the permeability is mainly governed by the diffusion coefficient. A large range of permeability values can thus be obtained by varying the nature and the content of the non-sulfonated diamine in the copolyimides and for some copolymers, H 2 and CO 2 permeability values are similar. Furthermore, O 2/N 2 selectivity coefficients determined for all the membranes are located near the Robeson curve. As a consequence, some systems, notably those based on (CARDO/ODA) diamines mixture, present a good selectivity/permeability balance.

Quantification of regional V/Q ratios in humans by use of PET. I. Theory.

With positron emission tomography, quantitative measurements of regional alveolar and mixed venous concentrations of positron-emitting radioisotopes can be made within a transaxial section through the thorax. This allows the calculation of regional ventilation-to-perfusion (V/Q) ratios by use of established tracer dilution theory and the constant intravenous infusion of 13N. This paper considers the effect of the inspiration of dead-space gas on regional V/Q and investigates the relationship between the measured V/Q, physiological V/Q, and V/Q defined conventionally in terms of bulk gas flow (VA/Q). Ventilation has been described in terms of net gas transport, and the term effective ventilation has been introduced. A simple two-compartment model has been constructed to allow for the reinspiration of regional (or personal) and common dead-space gas. By use of this model, with parameters representative of normal lung the effective V/Q ratio for 13N [(VA/Q)eff(13N)] is shown to overestimate VA/Q by 18% when VA/Q = 0.1 but underestimate VA/Q by 68% when VA/Q = 10. For physiological gases, the model predicts that the behavior of O2 should be similar to that of 13N, so that, in terms of gas transport, V/Q ratios obtained using the infusion of 13N closely follow those for O2. Values of the effective V/Q ratio for CO2 [(VA/Q)eff(CO2)] lie approximately halfway between (VA/Q)eff(13N) and VA/Q. These results indicate that dead-space ventilation is far less a confounding issue when V/Q is considered in terms of net gas transport (VAeff), rather than bulk flow (VA).(ABSTRACT TRUNCATED AT 250 WORDS)

Uptake of highly soluble gases in the epithelium of the conducting airways.

Short duration washin and washout experiments were carried out with the gases ether (diethyl ether), ethyl acetate and acetone in order to study the excretion behaviour in the lung of gases highly soluble in blood and tissues. Carbon dioxide (CO2) was analyzed as a reference gas during the washout. The excretion values for acetone were generally the lowest, those for ether the highest, and intermediate ones were obtained for ethyl acetate. These results are in accord with the experimental data of others. Analysis of the washout curves shows that for acetone the volume of gas expired before the beginning of phase II is considerably smaller than found for CO2 and ether. During washout, the slope of the alveolar plateau for acetone is negative for the first few breaths and becomes positive thereafter; however, it remains lower than the slopes for CO2 and ether, which are always positive. These two phenomena occurring during washout clearly demonstrate that the acetone in the expired air must originate from the epithelial tissue lining the conducting airways. We conclude, therefore, that, in terms of gas transport, the conducting airways behave differently for poorly and highly soluble gases and this provides a physiological basis for the deviating excretion behaviour of highly soluble gases.

Anomalous transport phenomena in semihydrophobic fuel cell electrodes

The so-called bubbling and weeping effects observed with porous semihydrophobic fuel cell electrodes are discussed in terms of gas transport in porous media. It is shown that the bubbling of gas on the electrolyte side can be accounted for by transport equations valid in the intermediate region between ordinary and Knudsen diffusion. The anomalous transport of liquid toward the gas side is attributed to isothermal distillation proceeding in the electrode pores. A gradient of the partial pressure of water vapour in the pores is the principal cause for both effects.