Alberto Caneiro

High Temperature Crystal Chemistry and Oxygen Permeation Properties of the Mixed Ionic–Electronic Conductors LnBaCo2O5 + δ ( Ln = Lanthanide )

The high temperature crystal chemistry and oxygen permeation properties of the cation-ordered LnBaCo 2 O 5+δ perovskite oxides [lanthanide (Ln) = Pr, Nd, and Sm] have been investigated in comparison with the cation-disordered La 0.5 Ba 0.5 CoO 3-δ perovskite. The LnBaCo 2 O 5+δ (Ln = Pr, Nd, and Sm) oxides exhibit a metal-insulator transition at T 350°C in air, as evidenced by an orthorhombic to tetragonal transition. At a given temperature, the oxygen permeation flux decreases from Ln = La to Nd to Sm due to the changes in crystal symmetry and lattice strain. The oxygen permeation mechanism in the Ln = Nd is bulk-diffusion-limited rather than surface-exchange-limited for membrane thickness L ≥ 1.1 mm.

Structural Stability and Oxygen Permeation Properties of Sr3 − x La x Fe2 − y Co y O 7 − δ ( 0 ⩽ x ⩽ 0.3 and 0 ⩽ y ⩽ 1.0 )

The structural stability and oxygen permeation properties of Sr 3-x La x Fe 2-x Co x O 7-δ with 0 ≤ x ≤ 0.3 and 0 ≤ y ≤ 1.0 have been studied at high temperature (800 ≤ T ≤ 900°C) and in the oxygen partial pressure range 10 5 ≤ pO 2 ≤ 0.21 atm. These phases have a perovskite-related intergrowth structure with tetragonal symmetry, which does not change with temperature up to 1000°C in the range 10 5 ≤ pO 2 ≤ 0.21 atm. The oxygen permeation flux of Sr 3-x La x Fe 2-x Co x O 7-δ membranes increases with increasing Co content, decreases with increasing Ianthanum content, and does not change with time. Measurements of oxygen permeation flux as a function of membrane thickness for Sr 2.7 La 0.3 Fe 1.4 Co 0.6 O 7-δ at 900°C indicates the oxygen transport is bulk limited for this composition. Assuming that this observation is also valid for other compositions, the ionic conductivity, σ 1 , and the vacancy diffusion coefficient D v have been estimated. The structural stability and permeation properties of Sr 3-x La x Fe 2-x Co y O 7-δ are compared with those of the perovskite phases SrFe 0.2 Co 0.8 O 3-δ and Sr 0.6 La 0.4 Fe 0.2 Co 0.8 O 3-δ .

A thermogravimetric analyser for corrosive atmospheres and its application to the chlorination of ZrO2-C mixtures

Abstract We report on a thermogravimetric analyser, built on the basis of a Cahn 2000 electrobalance, suitable for using with corrosive atmospheres. We discuss the corrections for buoyancy and gas flow effects, which strongly modify thermogravimetric curves. p]As an application, we studied the kinetics of a reaction between Cl 2 and a mixture of ZrO 2 and C. We were able to measure the uptake of Cl 2 by C and the reaction rate within the first 50 s. We present evidence of a transition to quite a different reaction rate at longer times.

Oxygen Reduction Reaction on Ruddlesden–Popper Phases Studied by Impedance Spectroscopy

The oxygen reduction mechanism of Ruddlesden-Popper phases Sr 3 FeMO 6+δ (M = Fe, Co, Ni) has been investigated by impedance spectroscopy at 500, 600, and 700°C under oxygen partial pressure pO 2 between 10 -5 and 1 atm using both He and Ar as gas carriers. Thick porous electrodes were sprayed on dense Ce 0.9 Gd 0.1 O 2-x and impedance spectra data were collected on symmetrical cells. An equivalent circuit was proposed considering the electrolyte resistances R el , a Warburg element W HF , and two parallel elements RCpe (RCpe IF and RCpe LF ). For the three compounds, W HF has been assigned to the oxygen vacancies diffusion in the bulk, the intermediate component, RCpe IF , to oxygen dissociative adsorption in the electrode surface, and the low frequency element, RCpe LF , to oxygen diffusion in the gas phases. In the case of the Sr 3 Fe 2 O 6+δ and Sr 3 FeCoO 6+δ compounds, the p0 2 dependence of Warburg high frequency component suggests a complex process involving both oxygen bulk diffusion and charge transfer. The results of Sr 3 FeMO 6+δ (M = Fe, Co, Ni) compared with those of La 0.6 Sr 0.4 Co 0.8 Fe 0.2 O 3-δ perovskite electrodes, allowing us to discuss the effect of the crystal structure on the electrochemical behavior of these layered compounds.

Emergence of ferromagnetism and Jahn-Teller distortion in LaMn1−xCrxO3 (x < 0.15)

The emergence of a ferromagnetic component in LaMnO3 with low Cr-for-Mn substitution has been studied by x-ray absorption spectroscopy and x-ray magnetic circular dichroism at the Mn and Cr K edges. The local magnetic moment strength for the Mn and Cr are proportional to each other and follows the macroscopic magnetization. The net ferromagnetic components of Cr3+ and Mn3+ are found antiferromagnetically coupled. Unlike hole doping by La site substitution, the inclusion of Cr3+ ions up to x = 0.15 does not decrease the Jahn-Teller (JT) distortion and consequently does not signi cantly a ect the orbital ordering. This demonstrates that the emergence of the ferromagnetism is not related to JT weakening and likely arises from a complex orbital mixing.

Tunneling barrier in nanoparticle junctions of La2/3(Ca,Sr)1/3MnO3: Nonlinear current–voltage characteristics

We study the nonlinear current–voltage (I–V) characteristics and analyze the voltage-dependent tunneling conductance in nanoparticles of La2/3A1/3MnO3 (A=Ca, Sr). The powders were prepared by different wet-chemical routes and low calcination temperatures were used to obtain an average particle size D≈30 nm. The data are comprehensively explained in terms of the tunneling picture, which allows one to estimate the height of the grain boundary insulating barrier (φ) for each sample. For constant D, our results show that the sample preparation route is mainly responsible for the value of φ in nanoparticles, while the Coulomb gap in the Coulomb blockade regime is ∼3 times higher for Sr- than for Ca-doping. We also show that a small fraction of the barriers contribute to the nonlinear transport, and the current is mainly carried through low-resistive percolated paths. In addition, despite the different barrier strengths, the low-field magnetoresistance (LFMR) is similar for all samples, implying that φ is not t...

Low-temperature oxygen electrode reaction on bismuth ruthenium oxides/stabilized zirconia

Abstract Bi2Ru2O7 and Bi3Ru3O11, have been tested as electrodes on stabilized zirconia at temperatures as low as 500 K. The oxides have been characterized by thermogravimetry under controlled oxygen partial pressure at high temperature and by potentiometric measurements after controlled cathodic polarization. The oxygen electrode reaction as functions of oxygen pressure and temperature has been studied by impedance spectroscopy. At low temperature, the dependence of the polarization resistance on the oxygen pressure was analyzed in terms of charge transfer as a rate controlling step.

Preparation and Characterization of Solid Oxide Fuel Cells Cathode Films

Perovskite oxide cobaltites with composition La0.4Sr0.6Co0.8Fe0.2O3-δ were prepared by an acetic acid-based gel route. Then, cathodes were deposited onto porous ceramic substrates, like the usual electrolytes Cerium Gadolinium Oxide (CGO), by spray, spin coating and dip-coating. The structure and morphology of the layers were characterized by XRD and SEM, respectively. Electrical properties were characterized by complex impedance measurements. The correlation between structural characteristics and electrical properties is discussed.

Oxygen vacancies effect on phase separation in Pr0.5Ca0.2Sr0.3MnO3−δ

We study the effect of the introduction of a controlled amount of oxygen vacancies on the magnetic and transport properties of the phase separated manganite Pr0.5Ca0.2Sr0.3MnO3−δ, for δ=0 and 0.01. This compound presents a paramagnetic to ferromagnetic (FM) transition below TC∼240 K, and then to a charge-ordered (CO) and antiferromagnetic (AFM) phase at TCO∼175 K. However, below TCO an appreciable FM component (∼20%) still survives, related to a FM volume immersed within the CO/AFM matrix. At low temperatures, in the mainly CO phase, the introduction of oxygen vacancies enhances the FM ordering, with the consequent decrease of resistivity and magnetoresistance (MR). On the contrary, in the FM phase (in the range TCO<T<TC) the magnetization is reduced and the resistivity and MR increase. These results are discussed in the frame of percolative transport in metal–insulator mixtures, and frustrated double exchange interaction due to the interruption of some Mn–O–Mn chains.

Local scale ordering in LaMnO3 under pressure: X-ray absorption study

The present study aims, by using X-ray Absorption Spectroscopy, to describe the local process towards the pressure-induced abrupt changes in the resistivity in the Jahn-Teller distorted LaMnO3 doped compounds. Local environment of manganese atoms under pressure up to 37 GPa has been studied by X-ray absorption spectroscopy. We identify successive steps towards the pressure-induced abrupt changes in the resistivity. These steps are associated to local processes such as the opening of the inter-octahedra tilt angles and the average symmetrization of the Mn local environment. All modifications present a hysteretic behavior when the pressure is released, pointing to the coexistence of MnO6 distorted and undistorted units.