Oxygen Transport Properties Research Articles

Melt crystallization of syndiotactic polypropylene in nanolayer confinement impacting structure

Layer multiplying coextrusion technique was used to fabricate films with hundreds of alternating layers of a crystallizable polymer, syndiotactic polypropylene (sPP), and an amorphous polymer, polycarbonate (PC). Atomic force microscopy and wide-angle X-ray scattering revealed the absence of any oriented crystal morphology of sPP in the extruded layered films. An approach of isothermal melt recrystallization of sPP nanolayers revealed the formation of oriented lamellae under the rigid confinement of hard glassy PC layers. X-ray scattering data showed that sPP crystallized as stacks of single crystal lamellae oriented parallel to the layers at high crystallization temperatures. As the crystallization temperature decreased, on-edge lamellar orientation was preferred. Formation of in-plane lamellae was attributed to heterogeneous bulk nucleation, while nucleation of on-edge lamellae was initiated at substrate interface. It was observed that as the layers thickness reduced, the orientations of both in-plane and on-edge lamellae became sharper. Detailed analysis of crystal orientations in 30 and 120 nm sPP layers was carried out. Melt recrystallization of 30 nm layers revealed formation of in-plane lamellae above 90 °C and mainly on-edge lamellae below 70 °C. At intermediate temperatures, formation of mixed crystals was reported. In 120 nm layers, crystallization temperature of 100 °C was required to form in-plane crystals, while on-edge lamellae were formed below 90 °C. We also investigated crystallization onset for on-edge and in-plane lamellar nucleation. Although, the two crystal fractions were significantly affected as a function of crystallization temperature, it was noticeable that both crystal habits were initiated at the same time. The results suggested that the relative growth rates of in-plane and on-edge crystal orientations was responsible for different fractions of the two crystal orientations at a given crystallization temperature. Oxygen transport properties of melt recrystallized sPP layers were measured. When the melt recrystallization temperature increased from 85 to 105 °C in 120 nm sPP layers, at least one order of magnitude enhancement in the barrier properties was observed. It was evident from the X-ray data that the amount of in-plane crystal fraction increased with increasing crystallization temperature. In-plane crystals acted as impermeable platelets to oxygen flux resulting in improved gas barrier properties. A similar effect was observed in 30 nm sPP layers over a temperature range of 60–105 °C. A correlation between in-plane crystal fraction and the oxygen permeability was obtained from X-ray and oxygen transport data analysis. It was shown that the permeability decreased exponentially with increasing in-plane crystal fraction.

Oxygen permeation and oxidation states of transition metals in (Fe, Nb)-doped BaCoO 3 − δ perovskites

The influence of cation substitution on the transition metal oxidation states, oxygen nonstoichiometry and oxygen transport properties of Ba(Co, Fe, Nb)O 3 − δ perovskites was investigated with room temperature XRD, X-ray absorption near edge structure and high temperature oxygen permeation measurements. Results show that substitution of Fe for Co is accompanied by a decrease in the lattice constant, whereas Nb substitution leads to the opposite trend. With increasing Fe and/or Nb content, the oxygen permeation flux decreases, whereas the apparent activation energy of the oxygen permeation increases. The oxidation state of Co is smaller than +3 and susceptible to the variation of cation composition. In contrast, the oxidation states of Fe and Nb are larger than +3 and close to +5, respectively, and almost independent of the composition. The influence of Fe and Nb contents on the oxygen transport properties is discussed in terms of the oxygen stoichiometry and metal–oxygen bonding.

Ln2MO4 cathode materials for solid oxide fuel cells

One of the major challenges to develop “intermediate temperature” solid oxide fuel cells is finding a novel cathode material, which can meet the following requirements: (1) high electronic conductivity; (2) chemical compatibility with the electrolyte; (3) a matched thermal expansion coefficient (TEC); (4) stability in a wide range of oxygen partial pressure; and (5) high catalytic activity for the oxygen reduction reaction (ORR). In this short review, a survey of these requirements for K2NiF4-type material with the formula Ln2MO4, Ln = La, Pr, Nd, Sm; M = Ni, Cu, Fe, Co, Mn, is presented. The composition-dependent TEC, electrical conductivity and oxygen transport property are considered. The Ln2MO4 materials exhibit improved chemical stability and compatibility with most of the traditional electrolytes. The complete fuel cells integrated with Ln2MO4 materials as cathodes show promising results. Furthermore, these materials are considered as cathodes of protonic ceramic fuel cell (PCFC), and/or anodes of high temperature steam electrolysis (HTSE). First results show excellent performances. The versatility of these Ln2MO4 materials is explained on the basis of structural features and the ability to accommodate oxygen non-stoichiometry.

The effects of thermal annealing on the structure and the electrical transport properties of ultrathin gadolinia-doped ceria films grown by pulsed laser deposition

Ultrathin crystalline films of 10 mol% gadolinia-doped ceria (CGO10) are grown on MgO (100) substrates by pulsed laser deposition at a moderate temperature of 400°C. As-deposited CGO10 layers of approximately 4 nm, 14 nm, and 22 nm thickness consist of fine grains with dimensions ≤∼11 nm. The films show high density within the thickness probed in the X-ray reflectivity experiments. Thermally activated grain growth, density decrease, and film surface roughening, which may result in the formation of incoherent CGO10 islands by dewetting below a critical film thickness, are observed upon heat treatment at 400°C and 800°C. The effect of the grain coarsening on the electrical characteristics of the layers is investigated and discussed in the context of a variation of the number density of grain boundaries. The results are evaluated with regard to the use of ultrathin CGO10 films as seeding templates for the moderate temperature growth of thick solid electrolyte films with improved oxygen transport properties.

The oxy‐hemoglobin dissociation curve of a semi‐fossorial rodent in the context of its burrow's hypercapnic environment

Animals, ranging in size from ants to aardvarks live in burrows. Burrow geometry, coupled with the comparatively low permeability of soils to gases, and the respiration of the burrow's tenants, may bring about fractional concentrations of CO2 (FCO2) that are well above those in the free atmosphere, especially when pups are present. Unless an animal is adapted to tolerate hypercapnia, increased inspired Pco2 usually leads to respiratory acidosis. Therefore, based on the hypothesis that semifossorial rodents are adapted to tolerate hypercapnia we tested the predictions that 1) large changes in inspired CO2 will result in only small changes in blood pH, 2) that the half saturation partial pressure of oxygen (P50) of a semi-fossorial rodent is relatively low, (in order to promote increased extraction of O2 from burrow air), and 3) that the ratio (Bohr factor) is relatively high, (to facilitate additional O2 unloading from hemoglobin to tissues). We investigated the blood oxygen transport properties of the semi-fossorial Sundevall's Jird (Meriones crassus) using the mixing method to trace the oxygen-hemoglobin dissociation curve. Under standard conditions (i.e. pH 7.37; T 37 °C; PCO2 40 mmHg), P50 averaged 42 mmHg and the Bohr factor 0.6. Both values are consistent with our predication when compared with values for other rodents.

Oxygen Exchange Kinetics of the IT-SOFC Cathode Material Nd2NiO4+δ and Comparison with La0.6Sr0.4CoO3-δ

For the promising intermediate temperature solid oxide fuel cell (IT-SOFC) cathode material Nd2NiO4+δ, chemical surface exchange coefficients kchem and chemical diffusion coefficients Dchem of oxygen have been determined by conductivity relaxation measurements and compared with results for La0.6Sr0.4CoO3-δ between 575 and 725°C. At 725°C and an oxygen partial pressure of 0.1 bar, kchem and Dchem of Nd2NiO4+δ amount to 1 × 10−3 cm s−1 and 2 × 10−6 cm2 s−1, respectively, which are higher than those for La0.6Sr0.4CoO3-δ. However, due to high activation energies, a strong decrease of both kinetic parameters is observed for Nd2NiO4+δ upon temperature reduction. Activation energies of kchem and Dchem are lower for La0.6Sr0.4CoO3-δ, leading to faster oxygen exchange compared to Nd2NiO4+δ at 600°C. Electronic conductivities of Nd2NiO4+δ amount to 100–125 S cm−1 while those of La0.6Sr0.4CoO3-δ are between 1600 and 2200 S cm−1 in the investigated temperature and oxygen partial pressure range. Ionic conductivities and surface exchange resistances, which were calculated from the kinetic parameters, further show superior oxygen transport properties of La0.6Sr0.4CoO3-δ compared to Nd2NiO4+δ in pure O2–Ar atmospheres.

A strategy to determine operating parameters in tissue engineering hollow fiber bioreactors

The development of tissue engineering hollow fiber bioreactors (HFB) requires the optimal design of the geometry and operation parameters of the system. This article provides a strategy for specifying operating conditions for the system based on mathematical models of oxygen delivery to the cell population. Analytical and numerical solutions of these models are developed based on Michaelis–Menten kinetics. Depending on the minimum oxygen concentration required to culture a functional cell population, together with the oxygen uptake kinetics, the strategy dictates the model needed to describe mass transport so that the operating conditions can be defined. If cmin ≫ Km we capture oxygen uptake using zero-order kinetics and proceed analytically. This enables operating equations to be developed that allow the user to choose the medium flow rate, lumen length, and ECS depth to provide a prescribed value of cmin. When , we use numerical techniques to solve full Michaelis–Menten kinetics and present operating data for the bioreactor. The strategy presented utilizes both analytical and numerical approaches and can be applied to any cell type with known oxygen transport properties and uptake kinetics.

A Two‐dimensional Network Study on Oxygen Transport in Porous Gas Diffusion Layer

AbstractOxygen transport in the porous gas diffusion layer (GDL), which is generally characterised by the oxygen effective diffusivity, is of great importance for the performance of proton exchange membrane fuel cells (PEMFCs). The determination of the oxygen effective diffusivity is challenging due to the complex structure of the porous GDL samples. In the present study, a two‐dimensional network consisting of arms and nodes is adopted to illustrate how oxygen effective diffusivity is affected by the GDL structure under the condition with/without water invasion. Water permeation in the network is simulated using the invasion percolation algorithm and oxygen transport in the arms is described by Fick's law. The simulation results reveal that oxygen effective diffusivity under dry condition decreases with increase in the network heterogeneity. With water permeation, the oxygen effective diffusivity goes to zero even though water saturation is rather less than unity. The critical water saturation, above which the oxygen effective diffusivity becomes zero, is found to decrease with increasing heterogeneity. To enhance oxygen transport, four different modified networks are introduced in the present study. It is found that the network with large arms in oxygen transport direction has the best oxygen and water transport properties.

Polydimethylsiloxane/liquid crystal cross-linked membranes: Preparation, characterization and oxygen transport properties

The introduction of liquid crystals (LCs) has presented an interesting approach to improving gas separation properties in polymeric membranes. A novel polydimethylsiloxane (PDMS)/LC cross-linked membrane was prepared with PDMS containing vinyl groups and LCs containing unsaturated linkages as matrix materials. The chemical structures and LC properties were characterized with FTIR, DSC and thermal polarized optical microscopy. Some influential factors that affect the permeability and transport properties were also studied. The results showed that the membranes exhibited good transport properties. The oxygen permeability coefficient and oxygen/nitrogen separation factor were as large as 789 Barrer and 3.40, respectively. The transport properties of the membrane are significantly larger than those of Robeson’s upper bound. Compared with modified PDMS membranes, the cross-linking method and the introduction of LCs are simple and convenient, and the transport properties are also enhanced.

Computation of some thermodynamic, structural, and transport properties of fluid oxygen using two-body and three-body intermolecular potentials from molecular dynamics simulation

We have performed molecular dynamics (MD) simulation to obtain pressure, internal energy, radial distribution function, and self-diffusion coefficient of fluid oxygen using effective two-site Lennard-Jones (2SLJ) and two-body Hartree–Fock dispersion (HFD)-like potentials. To take higher-body forces into account, we have used three-body potentials of Hauschild and Prausnitz [T. Hauschild, J.M. Prausnitz, Molec. Simul. 11 (1993) 177] and Miyano [Y. Miyano, Fluid Phase Equilib. 104 (1995) 71] with the two-body HFD-like potential. We have also considered the spin correction in our simulation which improved our energy results. The significance of this work is that the three-body potential of Hauschild and Prausnitz extended as a function of density and temperature and can be used with the HFD-like potential to improve the prediction of the pressures of fluid oxygen without requiring an expensive three-body calculation. The molecular dynamics simulation of oxygen has been also used to determine a new equation of state.

A simple model to predict the oxygen transport properties of multilayer films

A simple mathematical model to predict the oxygen permeability coefficient of multilayer films is presented in this work. Four multilayer systems and the corresponding monolayer films were tested: OPP/OPP (oriented polypropylene), PPcoex/PPcast (co-extruded and casted polypropylene, respectively), PET/PPcast (polyethylenterephthalate and casted polypropylene, respectively) and OPA/OPA/PE (oriented polyamide and polyethylene, respectively). To validate the model, oxygen permeability tests were conducted at 23 °C. In particular, the oxygen permeability coefficients of the monolayer films were firstly measured and then used to predict that of the multilayer film. This value was compared with that measured and the percentage error calculated. In the case of the PET/PPcast and OPA/OPA/PE systems, the oxygen barrier properties were measured and predicted at water activity ranging from 0% to 90%. Results showed that the percentage error between the experimental data and the predicted values by the model was close or below to 10% in most of the cases.

Phase transformation and oxygen equilibration kinetics of pure and Zr-doped Ba0.5Sr0.5Co0.8Fe0.2O3−δ perovskite oxide probed by electrical conductivity relaxation

Electrical conductivity relaxation is used for monitoring the cubic-to-hexagonal phase transition in the perovskite-type oxide Ba0.5Sr0.5Co0.8Fe0.2O3−δ. After in situ annealing at 1173 K, at which temperature the cubic polymorph is stable, electrical conductivity relaxation experiments are conducted during long-term annealing at 1073 K. The decreases in the surface exchange and chemical diffusion coefficients with time due to the sluggish phase transition are found to be correlated. It is demonstrated that B-site doping with Zr to an amount as low as 3 mol % stabilizes the high-temperature perovskite cubic phase of Ba0.5Sr0.5Co0.8Fe0.2O3−δ, with preservation of its high oxygen transport and exchange properties.

Oxygen permeability and stability of asymmetric multilayer Ba 0.5Sr 0.5Co 0.8Fe 0.2O 3 − δ ceramic membranes

Perovskite-type Ba 0.5Sr 0.5Co 0.8Fe 0.2O 3 − δ (BSCF) is considered as one of most promising materials for the oxygen separation from air. In order to assess the impact of asymmetric architecture on oxygen transport properties and stability, dense ceramics and membranes consisting of two porous and one dense layers were fabricated and tested. The stability of Ba 0.5Sr 0.5Co 0.8Fe 0.2O 3 − δ in atmospheres with low oxygen partial pressure is similar to that for SrCo 0.8Fe 0.2O 3 − δ . Analysis of the thickness dependence of oxygen permeation fluxes through symmetric dense membranes unambiguously showed that the permeation is limited by surface exchange kinetics at d ≤ 1 mm. The main limitations for the oxygen permeation through asymmetric membranes are associated with gas diffusion in pores and oxygen desorption process, despite a high open porosity (49%) of the porous layers. The asymmetric membranes demonstrated a stable performance if using inert sweep gas. On exposing to CO 2/H 2O containing atmospheres, the permeation decreases dramatically due to a deep decomposition process on the permeate side, resulting in the formation of carbonates and/or hydroxides causing blocking of oxygen transport.

Oxygen ion diffusion measurements in Bi 0.775La 0.225O 1.5

The oxygen transport properties of the Bi 0.775La 0.225O 1.5 electrolyte material have been investigated. Isotopic exchange depth profiling (IEDP) technique with secondary ion mass spectrometry (SIMS) was used in order to measure the oxygen tracer diffusion coefficient D. The activation energy for oxygen tracer diffusion was found to be 115 ± 2 kJ/mol (1.19 ± 0.02 eV). The measured D values were converted using the Nernst–Einstein relationship to conductivity and compared with data from AC Impedance. The agreement of the two sets of data implies that the material is an oxygen ion conductor.

Effects of humidified atmosphere on oxygen transport properties in La 2Mo 2O 9

The effects of humidified atmosphere on oxygen surface exchange and diffusion in La 2Mo 2O 9 have been investigated. After annealing samples in D 2O vapour, the depth and line scan profiles of the OD - species showed the incorporation of hydroxyl groups on the surface and of diffusion into the bulk. The hydroxyl diffusion process appears to be different from that of oxygen diffusion, and might indicate the existence of ambipolar diffusion of oxide ions and hydroxyl species in La 2Mo 2O 9.

BSCF epitaxial thin films: Electrical transport and oxygen surface exchange

Ba 0.5Sr 0.5Co 0.8Fe 0.2O 3 − δ (BSCF) shows promising oxygen transport properties for the use as oxygen separation membranes and cathode material for solid oxide fuel cells. In order to evaluate the electrical conductivity properties as well as surface exchange coefficients without the influence of grain boundaries present in polycrystalline bulk samples, high-quality epitaxial BSCF thin films on single crystal NdGaO 3 (110) (NGO) substrates were prepared for the first time. This was achieved by pulsed laser deposition (PLD) using single-phase cubic targets sintered from powder prepared via the mixed oxide route. Structural and compositional properties of the thin films were verified by various X-ray diffraction techniques as well as by scanning electron microscopy (SEM). The electrical conductivity of the thin films was measured in dependence of temperature and compared to values obtained by measurements on polycrystalline bulk samples from the same powder as well as data from literature, showing different activation energies and higher general conductivity. Dependence of the conductivity on oxygen partial pressure has also been studied, revealing a rather small variation and indicating a possible phase change beginning to occur at oxygen partial pressures below 10 − 4 bar. Additionally, the surface exchange coefficient k Chem of the purely surface exchange controlled samples was determined by a conductivity relaxation technique and yielded an activation energy of 93 kJ/mol.

Exchange kinetics and diffusion of oxygen in systems based on lanthanum gallate

The oxygen exchange and diffusion kinetics are studied by means of isotopic exchange in complex oxides La0.80Sr0.20Ga0.85 − x Mg0.15Co x O3 − δ (x = 0.00, 0.05, 0.15, 0.20, 0.25) and in electrolyte La0.88Sr0.12Ga0.82Mg0.18O3 − δ with gold-coated surface. The oxygen exchange rates and diffusion coefficients are determined in the bulk material and near the surface within the range of temperatures from 600 to 900°C and oxygen pressures from 0.3 to 1.0 kPa. The activation energies of the oxygen exchange and diffusion processes are calculated. By cobalt doping, gallium sublattice is shown to increase the interphase exchange rate; however, the oxygen transport properties in the bulk phase are not significantly affected by cobalt, while cobalt additive decreases the activation barrier of oxygen diffusion in the near-surface area. Gold activates the electrolyte surface, affecting the exchange rate and increasing the first-type exchange share. Cobalt substitution increases the third-type exchange share as compared to that of La0.88Sr0.12Ga0.82Mg0.18O3 − δ (x = 0.00, 0.05, 0.15, 0.20, 0.25) and in elec. The oxygen exchange with gas phase is assumed to be limited by molecular oxygen adsorption-desorption process on the electrolyte surface.

Oxygen Reduction Kinetics and Transport Properties of (Ba,Sr)(Co,Fe)O3-δ and Related SOFC Cathode Materials

Oxygen surface reduction kinetics is studied with the help of impedance spectra measured on dense thin film microelectrodes of (Ba1-xSrx)(Co1-yFey)O3-δ (BSCF) and related SOFC cathode materials. Effective oxygen exchange rate constants kq, calculated from area-specific surface resistances Rs extracted from the impedance spectra, increase in the sequence of LSM (La1-xSrxMnO3+/-δ), LSCF (La1-xSrxCo1-yFeyO3-δ) and BSCF. The influence of bulk properties, such as electronic conductivity and Goldschmidt tolerance factor, on kq is investigated to supply information for identifying the rate-determining step. Possible roles of oxygen vacancies in the oxygen surface incorporation reaction are discussed.

Oxygen transport properties of dense and porous (La 0.8Sr 0.2) 0.99Co 0.8Ni 0.2O 3-δ

We have determined k ex and D chem for (La 0.8Sr 0.2) 0.99Co 0.8Ni 0.2O 3-δ by the use of electrical conductivity relaxation on a dense sample and by applying the ALS model to measured AC impedance spectrum on a porous electrode. Extracting k ex and D chem from the methods resulted in comparable values. k ex and D chem also agreed well with literature values on La 0.8Sr 0.2CoO 3-δ, indicating that nickel substitution does not change the oxygen transport properties. k ex of the porous sample was further found to decrease with a five times higher rate than D chem when measured by using an Electrochemical Impedance Spectroscopy (EIS) over several days.

Synthesis and oxygen transport properties of La 0.2Sr 0.8Fe 1− xTi xO 3− δ ( x = 0.2, 0.4) intended for syn-gas production

Perovskites in the system La 0.2Sr 0.8Fe 1− x Ti x O 3− δ exhibit high stability at severe reducing conditions and high temperatures. Here we report the processing and oxygen transport properties of two compositions: La 0.2Sr 0.8Fe 1− x Ti x O 3− δ ( x = 0.2, 0.4). Single-phase powders were synthesized by spray pyrolysis (SP) and solid state reaction (SSR). The resulting average particle sizes (BET) ranged from 140 nm (SP) to 1090 nm (SSR). Samples were prepared by CIP and pressureless sintered between 1200 and 1250 °C for 8–12 h. Sintered densities ranged from 97 (SSR) to 99% (SP) and grain size was less than 1.5 μm. The average TEC varied in the range (13.8–14.6) × 10 −6 K −1 at 300–600 °C and (21.3–28.9) × 10 −6 K −1 at 600–800 °C. Surface exchange coefficients ( k chem ) and bulk diffusion coefficients ( D chem ) in terms of temperature and p O 2 were assessed by the method of electrical conductivity relaxation. A simple defect model was applied to explain electrical conductivity and bulk diffusion behaviour.