Oxygen transport in La0.5Sr0.5Fe1−yTiyO3−δ (y=0.0, 0.2) membranes

The influence of partial substitution of Fe with Ti on the oxygen transport properties of La_1−x Sr _x FeO₃ membranes was investigated in view of their application for oxygen separation. Samples of composition (y=0, 0.2) were prepared and their oxygen transport properties characterised by potential step relaxation and by oxygen permeation measurement in an air/argon gradient. With the first technique, chemical diffusion and surface exchange (k _S) coefficients were obtained by fitting of the current relaxation data to a single expression valid over the complete time range. The Ti-substituted composition gave slightly larger values of and k _S. The trend was opposite for the measured oxygen permeation flux. In the latter experience, ordering of oxygen vacancies was observed at lower temperature, reducing significantly the performance of the material.     

Recent developments in Ruddlesden–Popper nickelate systems for solid oxide fuel cell cathodes

Motivated by recent work on the Ruddlesden–Popper material, which was shown to be a superior oxide-ion conductor than conventional solid-oxide fuel cell cathode perovskite materials, we undertook A- and B-site doping studies of the Ruddlesden–Popper nickelate series in an attempt to identify other candidates for cathode application. In this paper, we summarize our most significant results for the and systems and more recently, the higher-order Ruddlesden–Popper phases La _n+1Ni _n O_3n+1 (n=2 and 3), which show greater promise as cathode materials than the n=1 compositions.     

Oxygen transport and thermomechanical properties of SrFe(Al)O3-δ –SrAl2O4 composites: microstructural effects

Measurements of oxygen permeation through dense composite membranes showed a considerable influence of processing conditions on the surface exchange kinetics, while the bulk ambipolar conductivity is almost unaffected by microstructural factors. Compared to the materials prepared via the glycine–nitrate process (GNP), the surface limitations to oxygen transport are significantly higher for dual-phase made of a commercial powder synthesized by spray pyrolysis. This difference in behavior may be related to compositional inhomogeneities in the grains of A-site deficient perovskite phase and an enhanced surface concentration of grain boundaries in the case of GNP-synthesized composite, which has also smaller grain size, slightly higher thermal expansion and lower total conductivity. No essential effects on Vickers hardness, varying in the range 6.3–6.5 GPa, were found. The deposition of porous catalyst layers onto the composite surface exposed to reducing environment leads to membrane decomposition. For the fabrication of tubular membranes, the cold isostatic pressing technique was, hence, combined with mechanical treatment to increase the specific surface area without incorporation of catalytically active components.     

Mixed-conducting dense ceramic membranes for air separation and natural gas conversion

Non-perovskite SrFeCo_0.5O _x (SFC2) was found to have high electronic and ionic conductivities as well as structural stability. At 800°C in air, total and ionic conductivities of 17 and 7 S·cm⁻¹ were measured, respectively; the ionic transference number was calculated to be ≈0.4. This material is unique because of its high electronic conductivity and comparable electronic and ionic transference numbers. X-ray diffraction analysis showed that air-sintered SFC2 consists of three phase components, ≈75 wt% , ≈20 wt% perovskite , and ≈5 wt% rock salt CoO. Argon-annealed SFC2 contains brownmillerite Sr₂(Fe_1−x Co _x )₂O₅ and rock salt CoO. Dense SFC2 membranes were able to withstand large pO₂ gradients and retain mechanical strength. A 2.9-mm-thick disk membrane was tested in a gas-tight electrochemical cell at 900°C; an oxygen permeation flux rate ≈2.5 cm³(STP)·cm⁻²·min⁻¹ was measured. A dense thin-wall tubular membrane of 0.75-mm thickness was tested in a methane conversion reactor for over 1,000 h. At 950°C, the oxygen permeation flux rate was ≈10 cm³(STP)·cm⁻²·min⁻¹ when the SFC2 thin-wall membrane was exposed with one side to air and the other side to 80% methane balanced with inert gas. Results from these two independent experiments agreed well. The SFC2 material is a good candidate as dense ceramic membranes for oxygen separation from air or for use in methane conversion reactors.     

Behavior of (La,Sr)CoO3- and La2NiO4-based ceramic anodes in alkaline media: compositional and microstructural factors

The behavior of dense ceramic anodes made of perovskite-type (x = 0.30–0.70; y = 0–0.05; z = 0–0.20) and K₂NiF₄-type (Me = Co, Cu; x = 0–0.20) indicates significant influence of metal hydroxide formation at the electrode surface on the oxygen evolution reaction (OER) kinetics in alkaline solutions. The overpotential of cobaltite electrodes was found to decrease with time, while cyclic voltammetry shows the appearance of redox peaks characteristic of Co(OH)₂/CoOOH. This is accompanied with increasing effective capacitance estimated from the impedance spectroscopy data, because of roughening of the ceramic surface. The steady-state polarization curves of in the OER range, including the Tafel slope, are very similar to those of model Co(OH)₂–La(OH)₃ composite films where the introduction of lanthanum hydroxide leads to decreasing electrochemical activity. La₂NiO₄-based anodes exhibit a low electrochemical performance and poor stability. The effects of oxygen nonstoichiometry of the perovskite-related phases are rather negligible at high overpotentials but become significant when the polarization decreases, a result of increasing role of oxygen intercalation processes. The maximum electrocatalytic activity to OER was observed for A-site-deficient , where the lanthanum content is relatively low and the Co⁴⁺ concentration determined by thermogravimetric analysis is highest compared to other cobaltites. Applying microporous layers made of template-synthesized nanocrystalline leads to an improved anode performance, although the effects of microstructure and thickness are modest, suggesting a narrow electrochemical reaction zone. Further enhancement of the OER kinetics can be achieved by electrodeposition of cobalt hydroxide- and nickel hydroxide-based films. Dedicated to Professor Dr. Yakov I. Tur’yan on the occasion of his 85th birthday.     

Properties and performance of BaxSr1−xCo0.8Fe0.2O3−δ materials for oxygen transport membranes

The present paper discusses the oxygen transport properties, oxygen stoichiometry, phase stability, and chemical and mechanical stability of the perovskites (BSCF) and (SCF) for air separation applications. The low oxygen conductive brownmillerite phase in SCF is characterized using in-situ neutron diffraction, thermographic analysis and temperature programmed desorption but this phase is not present for BSCF under the conditions studied. Although both materials show oxygen fluxes well above 10 ml/cm²·min at T=1,273 K and pO₂=1 bar for self-supporting, 200 μm-thick membranes, BSCF is preferred as a membrane material due to its phase stability. However, BSCF’s long-term stable performance remains to be confirmed. The deviation from ideal oxygen stoichiometry for both materials is high: δ>0.6. The thermal expansion coefficients of BSCF and SCF are 24×10⁻⁶ and 30×10⁻⁶ K⁻¹, respectively, as determined from neutron diffraction data. The phenomenon of kinetic demixing has been observed at pO₂<10⁻⁵ bar, resulting in roughening of the surface and enrichment with alkaline earth metals. Stress–strain curves were determined and indicated creep behavior that induces undesired ductility at T=1,073 K for SCF. Remedies for mechanical and chemical instabilities are discussed.     

Mechanical properties of mixed conducting La0.5Sr0.5Fe1−x Co x O3−δ (0≤x≤1) materials

Young’s modulus, strain–stress behavior, fracture strength, and fracture toughness of (0≤×≤1) materials have been investigated in the temperature range 20–1,000°C. Young’s moduli of and , measured by resonant ultrasound spectroscopy, were 130±1 and 133±3 GPa, respectively. The nonlinear stress–strain relationship observed by four-point bending at room temperature was inferred as a signature of ferroelastic behavior of the materials. Above the ferroelastic to paraelastic transition temperature, the materials showed elastic behavior, but due to high-temperature creep, a nonelastic respond reappeared above ∼800°C. The room temperature fracture strength measured by four-point bending was in the range 107–128 MPa. The corresponding fracture toughness of , measured by single edge V-notch beam method, was 1.16±0.12 MPa·m^1/2. The measured fracture strength and fracture toughness were observed to increase with increasing temperature. The fracture mode changed from intragranular at low temperature to intergranular at high temperature. Tensile stress gradient at the surface of the materials caused by a frozen-in gradient in the oxygen content during cooling was proposed to explain the low ambient temperature fracture strength and toughness.     

Amperometric biosensor for the determination of creatine

An amperometric biosensor for the determination of creatine was developed. The carbon rod electrode surface was coated with sarcosine oxidase (SOX) and creatine amidinohydrolase by cross-linking under glutaraldehyde vapour. The SOX from Arthrobacter sp. 1–1 N was purified and previously used for creation of a creatine biosensor. The natural SOX electron acceptor, oxygen, was replaced by an redox mediating system, which allowed amperometric detection of an analytical signal at +400-mV potential. The response time of the biosensor was less than 1 min. The biosensor showed a linear dependence of the signal vs. creatine concentration at physiological creatine concentration levels. The optimal pH in 0.1 M tris(hydroxymethyl)aminomethane (Tris)–HCl buffer was found to be at pH 8.0. The half-life of the biosensor was 8 days in 0.1 M Tris–HCl buffer (pH 8.0) at 20 °C. Principal scheme of consecutively followed catalytic reactions used to design a biosensor for the determination of creatine     

Effect of the mediator in feedback mode-based SECM interrogation of indium tin-oxide and boron-doped diamond electrodes

Indium tin-oxide (ITO) and polycrystalline boron-doped diamond (BDD) have been examined in detail using the scanning electrochemical microscopy technique in feedback mode. For the interrogation of electrodes made from these materials, the choice of mediator has been varied. Using ferrocene methanol (FcMeOH), and approach curve experiments have been performed, and for purposes of comparison, calculations of the apparent heterogeneous electron transfer rates (k _app) have been made using these data. In general, it would appear that values of k _app are affected mainly by the position of the mediator reversible potential relative to the relevant semiconductor band edge (associated with majority carriers). For both the ITO (n type) and BDD (p type) electrodes, charge transfer is impeded and values are very low when using FcMeOH and as mediators, and the use of results in the largest value of k _app. With ITO, the surface is chemically homogeneous and no variation is observed for any given mediator. Data is also presented where the potential of the ITO electrode is fixed using a ratio of the mediators and In stark contrast, the BDD electrode is quite the opposite and a range of k _app values are observed for all mediators depending on the position on the surface. Both electrode surfaces are very flat and very smooth, and hence, for BDD, variations in feedback current imply a variation in the electrochemical activity. A comparison of the feedback current where the substrate is biased and unbiased shows a surprising degree of proportionality.Dedicated to Alan, a good friend and colleague on his 60th birthday.     

On the largest eigenvalues of trees with perfect matchings

Let λ₁ (G) and Δ (G), respectively, denote the largest eigenvalue and the maximum degree of a graph G. Let be the set of trees with perfect matchings on 2m vertices, and . Among the trees in , we characterize the tree which alone minimizes the largest eigenvalue, as well as the tree which alone maximizes the largest eigenvalue when . Furthermore, it is proved that, for two trees T ₁ and T ₂ in (m≥ 4), if and Δ (T ₁) > Δ (T ₂), then λ₁ (T ₁) > λ₁ (T ₂).     

Palladized aluminum as a novel substrate for electrosynthesis of polyaniline in sulfuric acid solutions

The electropolymerization of aniline on a palladized aluminum electrode (Pd/Al) by potentiodynamic as well as potentiostatic methods is described. The effect of the monomer concentration between 0.01 and 0.4 M on the polyaniline (PANI) formation and its growth on the Pd/Al electrode was investigated and a suitable concentration of 0.2 M is suggested. A similar study was carried out to investigate the effect of sulfuric acid concentration and 0.1 M sulfuric acid was chosen. A study on the influence of electropalladization time on the polymer formation and its growth suggested a convenient time of 40 s. The stability of the PANI film on the Pd/Al electrode was studied as function of the potential imposed on the electrode. For applied electrode potentials of 0.1–0.7 V, the first-order degradation rate constant, k, of PANI film varies between 1×10⁻⁶ and 2×10⁻⁵ s⁻¹, and a relatively low slope (i.e., 2.2) was obtained for the plot of log k versus E. The coatings were characterized by scanning electron microscopy (SEM), and cyclic voltammetric behavior of the PANI-deposited Pd/Al electrode is discussed. The electrocatalytic activity of the PANI-deposited Pd/Al electrode against para-benzoquinone/hydroquinone (Q/H₂Q) and redox systems were investigated and on the basis of of the corresponding cyclic voltammograms and the redox systems were identified as the reversible and quasi-reversible systems, respectively.     

Interaction of glycine with cationic, anionic, and nonionic surfactants at different temperatures: a volumetric, viscometric, refractive index, conductometric, and fluorescence probe study

A number of thermodynamic parameters viz. apparent molar volumes, ϕ _v , partial molar volumes, , transfer volumes, , Falkenhagen coefficients, A, Jones–Dole coefficients, B, free energies per mole of solute, , and per mole of solvent, , molar refraction, R _D , and limiting molar conductivity, , have been calculated by using the experimentally measured densities, ρ, viscosities, η, refractive indices, n _D , and specific conductivities, κ, data of glycine (0.02–0.10 m) in 0.01 m aqueous sodium dodecyl sulphate, cetyltrimethylammonium bromide, and triton X-100 (TX-100) solutions at 298.15, 303.15, 308.15, and 313.15 K. The above calculated parameters were found to be sensitive towards the interactions prevailing in the studied amino acid–surfactant–water systems. Moreover, fluorescence study using pyrene as a photophysical probe has also been carried out, the results of which support the conclusions obtained from other techniques.     

Stable-isotope dilution activation analysis,and determination of Ca,Zn and Ce by means of photon activation

As a new method, stable-isotope dilution activation analysis has been developed. When an element consists of at least two stable isotopes which are converted easily to the radioactive nuclides through nuclear reactions, the total amount of the element (xg) can be determined by irradiating simultaneously the duplicated sample containing small amounts of either enriched isotope (y g), and by using the following equation. $${{x = y\left( {{M \mathord{\left/ {\vphantom {M {M*}}} \right. \kern-\nulldelimiterspace} {M*}}} \right)\left[ {\left( {{{R*} \mathord{\left/ {\vphantom {{R*} R}} \right. \kern-\nulldelimiterspace} R}} \right)\left( {{{\theta _2^* } \mathord{\left/ {\vphantom {{\theta _2^* } {\theta _2 }}} \right. \kern-\nulldelimiterspace} {\theta _2 }}} \right) - \left( {{{\theta _1^* } \mathord{\left/ {\vphantom {{\theta _1^* } {\theta _1 }}} \right. \kern-\nulldelimiterspace} {\theta _1 }}} \right)} \right]} \mathord{\left/ {\vphantom {{x = y\left( {{M \mathord{\left/ {\vphantom {M {M*}}} \right. \kern-\nulldelimiterspace} {M*}}} \right)\left[ {\left( {{{R*} \mathord{\left/ {\vphantom {{R*} R}} \right. \kern-\nulldelimiterspace} R}} \right)\left( {{{\theta _2^* } \mathord{\left/ {\vphantom {{\theta _2^* } {\theta _2 }}} \right. \kern-\nulldelimiterspace} {\theta _2 }}} \right) - \left( {{{\theta _1^* } \mathord{\left/ {\vphantom {{\theta _1^* } {\theta _1 }}} \right. \kern-\nulldelimiterspace} {\theta _1 }}} \right)} \right]} {\left[ {1 - \left( {{{R*} \mathord{\left/ {\vphantom {{R*} R}} \right. \kern-\nulldelimiterspace} R}} \right)} \right]}}} \right. \kern-\nulldelimiterspace} {\left[ {1 - \left( {{{R*} \mathord{\left/ {\vphantom {{R*} R}} \right. \kern-\nulldelimiterspace} R}} \right)} \right]}}$$ Where M and M^* are atomic weights of the element to be determined and the enriched isotope used as a spike,θ ₁ andθ ₂ are natural abundances of two stable isotopes in the element,θ ₁ ^* andθ ₂ ^* are isotopic compositions of the above isotopes in the enriched isotope, and R and R^* are counting ratios of gamma-rays emitted by two radionuclides produced in the sample and the isotopic mixture. Neither calibration standard nor correction of irradiation conditions are necessary for this method. Usefulness of the present method was verified by photon activations of Ca, Zn and Ce using isotopically enriched⁴⁸ca,⁶⁸Zn and¹⁴²Ce.     

Aqueous Geochemistry of Rare Earth Elements and Yttrium. XII: Potentiometric Stability Constant Determination of Bis-Tris Complexes with La, Nd, Eu, Gd, Yb, Dy, Er, Lu, and Y

Conditional stability constants of 2-[bis(2-hydroxyethyl)amino]-2(hydroxymethyl)-1,3-propanediol (BT) complexes of trivalent rare earth element (Ln) ions (La, Nd, Eu, Gd, Yb, Dy, Er, Lu) and Y were determined potentiometrically in aqueous NaCl solutions at 30°C and 0.1 M ionic strength. Least-squares fitting shows that, at <0.04 molal BT, the complex LnBT³⁺ is dominant, with LnBT₂ ³⁺ forming a secondary complex, where:

Mechanism of the catalyzed by cysteine electroreduction of the insoluble in water cobalt(II) salt as a component of carbon paste electrode

The mechanism of the Co(II) catalytic electroreduction of water insoluble CoR₂ salt in the presence of cysteine was developed. CoR₂ = cobalt(II) cyclohexylbutyrate is the component of a carbon paste electrode. Electrode surface consecutive reactions are: (a) fast (equilibrium) reaction of the complex formation, (b) rate-determining reversible reaction of the promoting process of CoR(Ac⁺) complex formation, (c) rate-determining irreversible reaction of the electroactive complex formation with ligand-induced adsorption, and (d) fast irreversible reaction of the electroreduction. Reactions (a,b) connected with CoR₂ dissolution and reactions (c,d) connected with CoR₂ electroreduction are catalyzed by . Regeneration of (reactions “b,d”) and accumulation of atomic Co(0) (reaction “d”) take place. Experimental data [Sugawara et al., Bioelectrochem Bioenergetics 26:469, 1991]: i _a vs E (i _a is anodic peak, E is cathodic accumulation potential), i _a vs , and i _a vs pH have been quantitatively explained.     

Doubly Bridged Dinuclear Cadmium(II)azido Schiff Base Complexes: Synthesis, X-Ray Structure and Luminescence Properties of [Cd(L)(N3)]2(Y)2 [L = Schiff Bases, Y = ClO4 −, PF6 −]

Three new Schiff bases, -(bis(pyridin-2-yl)alkylidene)butane-1,4-diamine [(py)(R)C=N-(CH)N=C(R)(py), L: py = pyridine; R = H, -(bis(pyridin-2-yl)formylidene)butane-1,4-diamine (bpfd); R = Me, -(bis(pyridin-2-yl)methylidene)butane-1,4-diamine (bpmd); R = C, -(bis(pyridin-2-yl)bezylidene)butane-1,4-diamine (bpbd)] were prepared and used to synthesize six dinuclear cadmium(II)azido complexes of type [Cd(L)(N)](Y) [L = bpfd, Y = ClO (1a); L = bpfd, Y = PF (1b); L = bpmd, Y = ClO (2a); L = bpmd, Y = PF (2b); L = bpbd, Y = ClO (3a); and L = bpbd, Y = PF (3b)]. Two representative members of the series, 3a and 3b, have been characterized by single crystal X-ray diffraction measurements. Structural study reveals that each cadmium center in both dinuclear compounds is located in a distorted octahedral coordination environment surrounded by six nitrogen atoms—four (N1, N2, N3, N4) from tetradentate ligand, and the fifth and sixth positions occupied by nitrogen atoms (N5, N5^*) of doubly end-on bridging azides. The complexes display intraligand ¹ fluorescence and intraligand ³ phosphorescence in glassy solutions (MeOH at 77 K).     

Synthesis and characterization of Naproxen intercalated Zn–Al layered double hydroxides

In this paper, naproxen was intercalated into Zn–Al layered double hydroxides (LDHs) by ion exchange method to obtain naproxen/LDHs nanohybrids. The effects of the contact time, the composition, and the structural charge density (σ _S,T ) and the specific surface area of LDHs, and pH value on the uptake of naproxen on LDHs, and the release of naproxen from the naproxen/LDHs nanohybrids were investigated. The adsorption isotherm curves of naproxen on the LDHs obey the Langmuir equation, and apparent monolayer capacity (A _m) in units of mmol m⁻² increases with the increase of the σ _S,T value of the LDHs samples. The release rate of naproxen from the naproxen/LDHs nanohybrids decreases with the increase of the σ _S,T value of the LDHs samples and is much lower than that of naproxen troche, indicating that the naproxen/LDHs nanohybrid is an efficient drug-controlled release system. In the pH range of 6~11.5, the uptake amount (A _eq) of naproxen on the LDHs decreases with the increase of pH value. The A _m values of LDHs(Cl⁻) are much higher than that of , which may contribute to that LDHs(Cl⁻), which has a stronger anion exchange ability than . The naproxen molecules are possibly adsorbed on each surface of the basal layer of LDHs. In other words, a bilayer is formed in the gallery of LDHs.     

Visualization technique for uncertainty budgets: Onion charts

A concise graphical depiction, the “onion” chart, is described for presenting uncertainty budgets for any measurand, y, having a set of component variances, . The onion chart consists of a concentric series of rings. Each ring is constructed from the set of fractional contributions, . The outer ring includes all and depicts the combined standard uncertainty, u _c (y), where u _c ²(y) ≡ ∑u _i ²(y). Moving inward, each successive ring deletes the largest remaining . A “reduced” is calculated for that ring from the remaining set of , with the fractional contributions calculated using the “reduced” . This format facilitates rapid visual comparison of the importance of each component, with minor components becoming visible as the larger components are successively deleted. An example is presented using data from pH metrology, including the physical interpretation of the resulting diagram.     

Oxygen permeation and stability of Zr0.8Y0.2O0.9‒La0.8Sr0.2CrO3-δ dual-phase composite

A tubular membrane made of Zr_0.8Y_0.2O_1.9 (60 vol%) and was prepared by a standard ceramic process. Oxygen permeation through the membrane tube was examined by exposing its outer shell to air and sweeping its inner wall with pure helium or CO balanced with helium. An oxygen flux of 9.2×10⁻⁹ mol cm⁻² s⁻¹ was measured at 950°C under air/He gradient, and a larger flux of 3.2×10⁻⁸ mol cm⁻² s⁻¹ at 930°C under air/CO gradient. The membrane tube was found to exhibit excellent stability under highly reducing atmosphere and elevated temperatures. The oxygen permeation rate is likely to be increased through the modification of the surface and reduction of the membrane thickness.     

Nanostructured electrochemical DNA biosensors for detection of the effect of berberine on DNA from cancer cells

Multi walled carbon nanotubes (MWNT) in dimethylformamide (DMF) or aqueous sodium dodecyl sulfate (SDS) solution, colloidal gold nanoparticles (GNP) in phosphate buffer solution (PBS), and a GNP–MWNT mixture in aqueous SDS solution have been investigated for chemical modification of a screen-printed carbon electrode used as the signal transducer of a dsDNA-based biosensor. Differential pulse voltammetry of the DNA redox marker and the guanine moiety anodic oxidation and cyclic voltammetry with K₃[Fe(CN)₆] as indicator revealed substantial enhancement of the response of the biosensor, particularly when MWNT in SDS solution was used. The biosensor was used in testing of berberine, an isoquinoline plant alkaloid with significant antimicrobial and anticancer activity. Berberine had a very strong, concentration-dependent, effect on the structural stability of DNA from the human cancer cells (U937 cells) whereas non-cancer cells were changed only when berberine concentrations were relatively high 75 and 50 μg mL⁻¹. Figure Schematic illustration of preparation of the nanostructured films: (a) layer-to-layer coverage (DNA/nanomaterial/SPE); (b) mixed coverage (DNA-nanomaterial/SPE)