Release and uptake of oxygen in mixed-conducting SOFC cathode materials measured by temperature-programmed methods

Oxygen uptake and release in A_1−aA′_aB_1−bB′_bO_3−δ perovskite-type oxides with A=La, Pr; A′=Sr; B=Fe, Mn, B′=Co was examined by means of temperature-programmed oxidation (TPO) and temperature-programmed reduction (TPR). The investigated powders were prepared by spray-drying and were annealed at 900 to 1300 °C. In the TPR profiles, two processes of oxygen release from these materials were observed. The first TPR peak was regarded as surface adsorption, while the second one was considered to result from oxygen bulk diffusion. The oxygen volumes involved and the activation energy for the oxygen diffusion were determined from the TPO and TPR profiles and varied between 0.53 and 2.2 eV for the ferrites and manganites, respectively. Paper presented at the 9th EuroConference on Ionics, Ixia, Rhodes, Greece, Sept. 15–21, 2002.     

Phase transformation and oxygen permeation properties of partially substituted strontium cobaltite — Influences of B-site partial substitution —

A group of B-site partially substituted strontium cobaltites Sr_0.9Ca_0.1Co_0.9B'_0.1O_2.5+δ (B'=Mn, Co, Ni, In, Fe and Al) were examined for phase transformation and oxygen permeation properties. As revealed by XRD analysis, these oxides had 2H-BaNiO₃-type structure (B'=Mn, Co, Ni and In) or brownmillerite-type structure (B'=Fe and Al) and transformed into perovskite-type structure at elevated temperatures (750 °C or above). The transformation temperature was dependent on the B-site substituent and agreed well with the switching-on temperature of oxygen permeation for the same oxide. Due to a lowering in transformation temperature, the Fe- or Al-doped oxide was made oxygen permeable in the lower temperature range 750–850 °C where the undoped oxide remained non-permeable. It was also found that for a series of Fe-partially substituted samples Sr_0.9Ca_0.1Co_1−xFe_xO_2.5+δ the transformation temperature lowered with increasing x up to 0.1, while, for x≥0.2, perovskite-type structure tended to be stabilized at room temperature. TGDTA analysis revealed that phase transformation was accompanied by absorption or desorption of oxygen. Based on the correlation between the oxygen permeability and the membrane thickness, the rate-determining step of oxygen permeation in the present system was assumed to be of the bulk diffusion of oxide ion vacancies.     

Impedance spectroscopy study of BNKLT polycrystalline ceramic

Complex impedance analysis of perovskite structured polycrystalline, [Bi_0.5(Na_1−x−y K _x Li _y )_0.5]TiO₃, at x=0.2, y=0.1 ceramic was synthesized by a mixed oxide method. The formation of single-phase material was confirmed by X-ray studies, and it was found to be rhombohedral structure at room temperature. Under scanning electron microscope, grains separated by well-defined boundaries are visible, which is in good agreement with impedance analysis. The BNKLT ceramic shows excellent piezoelectric properties and the optimum properties measured are: d ₃₃=251 pC/N, g ₃₃=24×10⁻³ mV/N, k _p =30.5% and k _t =28.1%. A complex impedance spectroscopy (CIS) study has been carried out to investigate the electrical properties. Impedance and modulus plots helped to separate the grain and grain boundary to the overall polarization or electrical behavior. CIS analysis suggests the presence of temperature-dependent relaxation process in the material. A possible hopping mechanism for electrical transport processes in the studied material is evident from the modulus analysis. The modulus mechanism indicates the non-Debye type of conductivity relaxation in the materials, which is supported by impedance data. The activation energies have been calculated from impedance (E _τ =0.58 eV) and electric modulus (E _τ =0.40 eV) studies, which suggests that the conduction is ionic in nature. The variation in width of the curves, M"/M"_maxM'/M'_{\max} and Z"/Z"_maxZ'/Z'_{\max} at FWHM, allows to conform that the relaxation process involved is of non-Debye type.     

A comparison of the transport properties of La2−xSrxNi1−yFeyO4+δ where 0<x<0.2 and 0<y<0.2

Recently there has been substantial interest in optimising perovskite type ceramics as mixed ionic-electronic conductors (MIECs) for use in ceramic oxygen generators and solid oxide fuel cells. However these materials suffer from thermomechanical deficiencies and therefore there is a need to develop alternative materials. Using the IEDP/SIMS technique the La_2−xSr_xNi_1−yFe_yO_4+δ series of compounds has been investigated and the oxygen tracer diffusion and surface exchange coefficients determined. It has been found that the oxide ion diffusivity of the x and y=0 material is very close to that of the fast oxide ion conducting perovskites such as La_1−xSr_xCoO₃ (LSC) but on acceptor doping with Sr a considerable decrease in the oxygen tracer diffusion coefficient was observed. Further studies on the effects of B site doping, where y>0, indicate that this has very little effect on the transport properties. Paper presented at the 6th Euroconference on Solid State Ionics, Cetraro, Calabria, Italy, Sept. 12–19, 1999.     

Electronic energy of the <Superscript>1</Superscript><Emphasis Type="Italic">L</Emphasis><Subscript>a</Subscript> level of supersonic jet-cooled indole

The dependence of the fluorescence polarization degree on frequency upon excitation into the first and second electronic bands of jet-cooled indole has been studied. A structured band consisting of a number of lines rather than a single line has been recorded in the spectral range of the electronic ¹A′-¹L_a-transition. The appearance of the lines is interpreted as the result of nonadiabatic vibronic interaction between the electronic ¹L_a-state and vibronicl states of the indole molecule ¹L_b-level. The energy of the unperturbed electronic ¹L_a-level and the orientation of dipole moments near the ¹A′-¹L_a-transition have been found. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 75, No. 3, pp. 319–323, May–June, 2008.     

Photoluminescence Enhancement Effect of CeO2 in Rare Earth Composites MM′O3/CeO2 and MM′O3/CeO2: Pr3+ (M = Ca, Sr; M′ = Ti, Zr)

In the context, a modified sol-gel technology was afford to the synthesis of rare earth composite ceramic phosphors MM′O₃/CeO₂ and MM′O₃/CeO₂: Pr³⁺ (M = Ca, Sr; M′ = Ti, Zr) with multicomponent hybrid precursors were composed. The micromorphology, particle size and photoluminescence properties were studied with XRD, SEM and luminescent spectroscopy in detail. Both XRD and SEM indicated the particle sizes were in the submicrometer range of 100 ∼ 300 nm. The photoluminescence for these ceramic phosphors were studied in details with the different component of host (molecular ratio of Sr, Ca and Ti, Zr), presenting a broad spectral band in the visible blue-violet region with the maximum excitation peak at 449 nm and a wide emission range with a maximum peak at 619 nm, which was ascribed to be the characteristic transition of Pr³⁺ (¹D₂ → ³H₄). These phosphors can be expected for visible light conversion (blue → red) materials. Especially it can be found that the introduction of CeO₂ can enhance the luminescence intensity of MM′O₃ and MM′O₃: Pr³⁺.     

Interface optical anisotropy in a heterostructure with different cations and anions

A theory of optical anisotropy and quantum-confined Pockels effect in CA/C′A′(001) quantum-well structures with different cations and anions, i.e. for C≠C′ and A≠A′, has been developed. The theory is based on a generalized effective-mass method, in which the boundary conditions for the envelope functions were constructed taking into account the mixing of heavy-and light-hole states under normal incidence of the hole on the interface. It is shown that an absorption anisotropy in interband transitions occurs for different mixing coefficients t _l−h in the boundary conditions for the right-hand (A-C′) and left-hand (A′-C) interfaces. An analysis is made of the interface contribution to the anisotropy induced by an external electric field for coinciding and different band offsets at the interfaces. The microscopic sp ³ s* tight-binding model is used to estimate the difference between the t _l−h(A-C′) and t _l−h(A′-C) coefficients. Fiz. Tverd. Tela (St. Petersburg) 40, 1925–1931 (October 1998)     

Molecular oxygen detection in low pressure flames using cavity ring-down spectroscopy

Cavity ring down spectroscopy is used for measurement of the concentration profiles of oxygen in the low pressure (30 Torr) methane/nitrogen/oxygen flames. Three different equivalence ratios are used: 0.8, 1.0 and 1.17. Molecular oxygen concentration is monitored via rotational spectrum of b¹ Σ _g ⁺←X³ Σ _g ^- (v^′=0-v^′′=0) transition, also known as atmospheric A band, located near 750 nm. The P(15)P(15) line is used for concentration measurements. The sensitivity reached is 2.2×10^-8 cm^-1. The concentration profiles are in a good agreement with the ones calculated using GRI-3.0 mechanism. PACS 33.20.Kf; 33.70.Fd; 42.60.Da     

Critical behavior of the specific heat of small magnetic Cr2O3 particles

The critical behavior of the specific heat of small magnetic particles of the real antiferromagnet Cr₂O₃ is investigated by the Monte-Carlo method. The critical indices α=−0.17±0.03 and the ratio of the critical amplitudes A/A′=1.03±0.07 are calculated for particles containing N=286 to 2502 spins. Fiz. Tverd. Tela (St. Petersburg) 40, 1661–1662 (September 1998)     

Collisional processes near the CH B 2Σ- v′=0,1 predissociation limit in laser-induced fluorescence flame diagnostics

Excitation and dispersed laser-induced fluorescence spectra of CH B ²Σ^-v^′=0,1 in methane flames are analyzed using rotational relaxation models to investigate their applicability for flame diagnostics. The existence of non-predissociative and highly predissociative rotational levels in the same vibrational state provides a unique scenario to test the effects of rotational relaxation in laser-induced fluorescence measurements. Using a statistical power gap law for rotational relaxation modeling, we find that the levels with collision-free lifetimes as short as 100 ps have apparent fluorescence yields larger than expected because of the extent of rotational relaxation at atmospheric pressure. Also, vibrational (v^′=1 to v^′=0) and electronic energy transfer (B ²Σ^-v^′=1 to A ²Δ) are competitive, and together are half the value for the total collisional removal rate from CH B ²Σ^-v^′=0. The measured electronic energy transfer branching ratio into A (v^′=0-3) depends on the initial rotational level pumped, and energy gap considerations can be used to explain these propensities. The combination of measurements and model calculations finds the excitation of the CH B ²Σ^- v^′=1,N^′=8 level a good candidate for laser-induced fluorescence quantitative measurements in flames at atmospheric pressure. Received: 24 September 1999 / Published online: 7 June 2000     

Measurements of the temperature and water vapor concentration in a hot zone by tunable diode laser absorption spectrometry

A tunable diode laser absorption spectroscopy (TDLAS) technique and appropriate instrumentation was developed for the measurement of temperature and water vapor concentrations in heated gases. The technique is based on the detection of the spectra of H₂O absorption lines with different energies of low levels. The following absorption lines of H₂O were used: 7189.344 cm⁻¹ (E″=142 cm⁻¹), 7189.541 cm⁻¹ (E″=1255 cm⁻¹), 7189.715 cm⁻¹ (E″=2005 cm⁻¹). Spectra were recorded using fast frequency scanning of a single distributed feedback (DFB) laser. A unique differential scheme for the recording of the absorption spectra was developed. An optimal technique for fitting the experimental spectra was developed.     

Fast magnetic relaxation, studied by microwave absorption in neutron-irradiated YBa2Cu3O7−x ceramic

A contactless microwave method is used to measure the fast magnetic relaxation of granular ceramic samples of YBa₂Cu₃O_7−x irradiated by neutrons with fluences of 10¹⁶–10¹⁹ cm⁻². An experimental study of the time dependence of the relaxation of high-frequency (rf) absorption (f=100 MHz) after the action of an external magnetic field pulse has shown the magnetic relaxation times τ ₀ to be in the time interval of 0.5–150 ms. The rf-absorption mechanism is discussed in terms of an intergranular system with a thermally activated flux of vortices and their diffusion in the granular medium. Fiz. Tverd. Tela (St. Petersburg) 39, 977–981 (June 1997)     

Running coupling: does the coupling between dark energy and dark matter change sign during the cosmological evolution?

In this paper we put forward a running coupling scenario for describing the interaction between dark energy and dark matter. The dark sector interaction in our scenario is free of the assumption that the interaction term Q is proportional to the Hubble expansion rate and the energy densities of dark sectors. We only use a time-variable coupling b(a) (with a the scale factor of the universe) to characterize the interaction Q. We propose a parametrization form for the running coupling b(a)=b ₀ a+b _e (1−a) in which the early-time coupling is given by a constant b _e , while today the coupling is given by another constant, b ₀. For investigating the feature of the running coupling, we employ three dark energy models, namely, the cosmological constant model (w=−1), the constant w model (w=w ₀), and the time-dependent w model (w(a)=w ₀+w ₁(1−a)). We constrain the models with the current observational data, including the type Ia supernova, the baryon acoustic oscillation, the cosmic microwave background, the Hubble expansion rate, and the X-ray gas mass fraction data. The fitting results indicate that a time-varying vacuum scenario is favored, in which the coupling b(z) crosses the noninteracting line (b=0) during the cosmological evolution and the sign changes from negative to positive. The crossing of the noninteracting line happens at around z=0.2–0.3, and the crossing behavior is favored at about 1σ confidence level. Our work implies that we should pay more attention to the time-varying vacuum model and seriously consider the phenomenological construction of a sign-changeable or oscillatory interaction between dark sectors.     

Phase equilibria in the La-Me-Co-O (Me=Ca, Sr, Ba) systems

The phase equilibria of the La-Me-Co-O systems (Me = Ca, Sr and Ba) were studied in air at 1100 °C. Two types of solid solution of general composition La_1−xMe_xCoO_3−δ and (La_1−y Me_y)₂CoO₄ were found to exist in the systems. The limiting composition of La_1−xMe_xCoO_3−δ lies at x=0.8 for Me = Sr, Ba and between 0.3–0.5 for Me = Ca. It is shown that the rhombohedral distortion of the perovskite type La_1−xMe_xCoO_3−y decreases while x increases. La_1−xMe_xCoO_3−δ (Me = Sr, Ba) shows an ideal cubic structure at x=0.5. The stability range of (La_1−yMe_y)₂CoO₄ was found to be 0.25≤y≤0.35 for Me = Ca, 0.3≤y≤0.55 for Me = Sr and 0.3≤y≤0.375 for Me = Ba. All phases have tetragonal K₂NiF₄-type crystal structure. Based on the XRD and neutron diffraction patterns of quenched samples, the phase diagrams (Gibbs triangles) are constructed for all systems. The phase equilibrium at low oxygen pressure is shown for the example of the La-Sr-Co-O system. The decomposition mechanism of La_1−xSr_xCoO_3−δ at 1100 °C for the samples with 0.5<x<0.8 within the oxygen pressure range −0.678>log(Po₂)>−2.25 can be written as follows: La_1−x′ Sr_x′CoO_3−δ′=n La_1−x″Sr_x″CoO_3−δ″+m SrCoO_2.5+q/2 O₂ where x′>x″. The decomposition mechanism of La_1−xSr_xCoO_3−δ for the samples with x < 0.5 within the oxygen pressure range −2.25>log(Po₂)>−3.55 changes and can be written as follows: La_1−xSr_xCoO_3−δ′=r La_1−x′Sr_x′CoO_3−δ″+w (La_1−y′Sr_y′)₂CoO₄+v CoO+f/2 O₂. The results are shown in “logPo₂-composition” diagrams. Paper presented at the 5th Euroconference on Solid State Ionics, Benalmádena, Spain, Sept. 13–20, 1998.     

A theoretical model for the collective motion of proteins by means of principal component analysis

A coarse grained model in the frame work of principal component analysis is presented. We used a bath of harmonic oscillators approach, based on classical mechanics, to derive the generalized Langevin equations of motion for the collective coordinates. The dynamics of the protein collective coordinates derived from molecular dynamics simulations have been studied for the Bovine Pancreatic Trypsin Inhibitor. We analyzed the stability of the method by studying structural fluctuations of the C _a atoms obtained from a 20 ns molecular dynamics simulation. Subsequently, the dynamics of the collective coordinates of protein were characterized by calculating the dynamical friction coefficient and diffusion coefficients along with time-dependent correlation functions of collective coordinates. A dual diffusion behavior was observed with a fast relaxation time of short diffusion regime 0.2–0.4 ps and slow relaxation time of long diffusion about 1–2 ps. In addition, we observed a power law decay of dynamical friction coefficient with exponent for the first five collective coordinates varying from −0.746 to −0.938 for the real part and from −0.528 to −0.665 for its magnitude. It was found that only the first ten collective coordinates are responsible for configuration transitions occurring on time scale longer than 50 ps.     

Systematic trends in the line strengths and radial matrix elements of E1 transitions in the Be isoelectronic sequence

Line strengthS and radial matrix elementσ for the dipole allowed transitions withinn=2 complex of ions in the Be isoelectronic sequence have been fitted in the formsZ ² S=A+B/(Z − C) andZσ=A′ + B′/(Z − C′). The constantsA, B, C andA′, B′, C′ have been calculated by employing a non-linear least square method. The relevant data forS andσ have been taken from calculations which includes correlation effects. It is shown that the fitted yalues ofA andA′ are in excellent accord with their hydrogenic values (Z →α) provided that we express the zeroth-order wavefunction of the ground state 1s²2s^{2 1} S as a quantum-mechanical admixture of the Hartree-Fock (HF) state 1s²2s^{2 1} S and the near-degenerate state 1s²2p^{2 1} S.     

Spectral Estimates for Periodic Jacobi Matrices

 We obtain bounds for the spectrum and for the total width of the spectral gaps for Jacobi matrices on ℓ²(ℤ) of the form (Hψ) _n =a _n−1 ψ _n−1 +b _nψ _n +a _nψ _n+1 , where a _n=a _n+q and b _n=b _n+q are periodic sequences of real numbers. The results are based on a study of the quasimomentum k(z) corresponding to H. We consider k(z) as a conformal mapping in the complex plane. We obtain the trace identities which connect integrals of the Lyapunov exponent over the gaps with the normalised traces of powers of H. Received: 17 April 2002 / Accepted: 1 October 2002 Published online: 13 January 2003 Communicated by B. Simon     

<Superscript>1</Superscript>H Spin–Lattice Relaxation Study of Dynamical Inequivalence of Methyl Groups in Solid 1,2-<Emphasis Type="Italic">O</Emphasis>-(1-Ethylpropylidene)-α-<Emphasis Type="SmallCaps">d</Emphasis>-Glucofuranose

We investigated the dynamics of methyl groups in organic polycrystalline 1,2-O-(1-ethylpropylidene)-α-d-glucofuranose by the proton spin–lattice relaxation method. The temperature and nuclear magnetic resonance Larmor frequency dependence of relaxation time is presented and interpreted in terms of simple possible dynamical model for the reorientation of methyl groups: the random hopping for methyl groups, which are in a, b, and c sites in the crystal. The energy E _a of 13.5 kJ mol⁻¹ for the a-type methyl groups is typical for methyl groups in ethyl groups. In contrast, the b- and c-methyl groups characterized by the lower E _a values of 9.5 and 6.5 kJ mol⁻¹ are located in the crystal structure where the intermolecular interactions significantly influence the potential, leading to a decrease in the total energy.     

μ− SR studies of oxide-superconductorsSR studies of oxide-superconductors

The negative muon spin rotation method (μ⁻ SR) has been applied to studies of electronic states at oxygen sites of oxide superconductors YBa₂Cu₃O₇, Nd_2−x Ce _x CuO_4−δ (x=0.15, oxygen reduced), LiTi₂O₄ and related oxide-insulators La₂CuO_4−δ, CuO, Cu₂O. The paramagnetic shifts of μ⁻ trapped at oxygen nuclei in these polycrystalline powder samples have been measured at 300 K. All the measured shifts are positive. In copper-oxides the paramagnetic shifts are of the order 10⁻³, while in LiTi₂O₄ is very small (8.4±3.34×10⁻⁵). In YBa₂Cu₃O₇, a fast μ⁻ spin relaxation timeT ₂ ^* (∼ 200 ns) has been observed; the reason for this is unknown and further studies are now in progress.     

Anisotropy of rotation of nitroxide probes in nematogenic liquid crystals

An electron paramagnetic resonance (EPR) line shape simulation for nitroxide spin probes in the motional narrowing region was carried out assuming axially symmetricg andA tensors and with different anisotropies of rotationN (=R _∥/R _⊥) whereR _∥ andR _⊥ are, respectively, elements of the diffusion tensor along and perpendicular to its principal axisz′. In addition, it was assumed that the principal axes of the diffusion tensor coincide with the molecular axes. Each of three casesz′=x,z′=y andz′=z, which result from cyclic permutations of the molecular axesx, y andz with thez′,y′ andx′ axes of the diffusion tensor, yields its typical EPR spectrum characterized by the relative intensities of the low-, center- and high-field lines. The parameter δ defined by and calculable from the intensities of the three lines was found to vary linearly withN for thez′=x andz′=y cases and, as anticipated, to be practically constant at a value of 1 for thez′=z case. This suggested a method for estimatingN for a probe from its EPR spectrum. Experimental spectra over a narrow temperature range (1°C) in the vicinity of the nematic-to-isotropic transition (about 34.6°C) ofN-(4-n-butylbenzilidene)-4-amino-2,2,6,6-tetramethylpiperidine-1-oxide at a mole fraction of 1·10⁻³ in 4-n-pentyl-4′-cyanobiphenyl showed a pattern of peak heights characteristic of thez′=x case with δ values that gave, neglecting effects of the mean field, higher and lowerN values in the nematic and isotropic regions, respectively. Analysis of other similar systems in the literature gave similar results.