Oxygen reduction at sol–gel derived La0.8Sr0.2Co0.8Fe0.2O3 cathodes

The perovskite material, La_0.8Sr_0.2Co_0.8Fe_0.2O₃ (LSCF), substituted by Sr and Fe at the A and B sites, was prepared using the sol–gel (SG) method, followed by heating at 900 °C for 4 h. The X-ray powder diffraction pattern for the SG derived LSCF material showed good agreement with the literature data. Scanning electron microscopy showed that the LSCF structure is highly porous, facilitating gas transfer and maximizing the number of active sites for the oxygen reduction reaction (ORR) at the cathode of a solid oxide fuel cell. Transmission electron microscopy (TEM) was employed to determine the SG–LSCF particle size and distribution. The kinetics of the ORR were investigated at SG–LSCF, deposited by screen-printing on a samarium-doped ceria (SDC) electrolyte, using electrochemical impedance spectroscopy and cyclic voltammetry at temperatures ranging from 400 to 700 °C. The results showed that the SG–LSCF cathode is stable and exhibits a high exchange current density (and low charge transfer resistance), yielding an apparent activation energy for the ORR of ca. 120 kJ/mol. It was also found that the SG–LSCF on SDC cathode was approximately one order of magnitude more active than standard manganite-based composite cathodes, deposited on yttria stabilized zirconia, studied under otherwise identical operating conditions.     

High temperature water electrolysis: Cathodic behaviour of Pt/yttria stabilized zirconia (ysz) interface

The electrolysis of steam using a solid state cell fed by mixtures of hydrogen and water has been carried out at temperatures ranging 700 to 1000°C. The kinetics of the cathodic reduction of water at Pt/YSZ interface has been investigated.Two different processes seem rate determining depending upon the current densities. The transition between each other was found to depend on the temperature but not on the hydrogen vs. water ratio of the feeding mixtures. At low current densities the over-voltage shows a typical Tafel behaviour; the activation enthalpy of the cathodic process was evaluated around 14 Kcal/mole. At high current densities the straight line relationship between total applied voltage and current density suggests the process to occur without apparent overvoltage. This behaviour has been explained in terms of formation of large amounts of complex defects acting as charge exchange sites for the water reduction.The faradic yield of the electrolysis has been proved to be 100%.     

Synthesis and investigations on the stability of La0.8Sr0.2CuO2.4+δ at high temperatures

For application in solid oxide fuel cells La_0.8Sr_0.2CuO_2.4+δ was synthesized and the phase evolution was characterized after quenching from different temperatures and after slow cooling. A single phase perovskite was found after quenching from 950 °C. The electrical conductivity of the La_0.8Sr_0.2CuO_2.4+δ perovskite exhibited metallic behavior reaching values of about 270 S/cm at 800 °C in air. The thermal expansion between 30 and 800 °C gave a thermal expansion coefficient of 11.1 × 10^− 6 K^− 1.At higher temperatures, the perovskite was transformed to the K₂NiF₄-type structure via an intermediate stage that can be best described as a LaSrCuO₄ phase with preferential growing of {020} lattice planes. After sintering at 1100 °C and slow cooling in the furnace a phase mixture of (La,Sr)CuO_4+δ and (La,Sr)CuO_2.4+δ perovskite was obtained. This phase mixture showed higher electrical conductivity (400 S/cm at 800 °C) and smaller thermal expansion coefficient (9.6 × 10^− 6 K^− 1) than the single phase La_0.8Sr_0.2CuO_2.4+δ perovskite.     

Ho3+掺杂的Ba0.8Sr0.2TiO3厚膜的发光特性

用丝网印刷法在氧化铝衬底上制备了Ho3+掺杂的Ba0.8Sr0.2TiO3(BST)厚膜.XRD谱表明,随着Ho3+掺杂摩尔分数从0.5%增加到2.0%,BST厚膜的晶格尺寸先增大后减小.在514 nm的氩离子激光激发下,样品均出现了中心波长为549,650,753 nm的发光带,分别对应于Ho3+离子的(5F4,5S...     

Response of La0.8Sr0.2CoO3-δ to perturbations on the CoO3 sublattice

Emission and transmission M?ssbauer studies of La_0.8Sr_0.2CoO_3-δ perovskites doped with ∼0.02 stoichiometric units of oxygen vacancy or 2.5% iron corroborate the occurrence of electronic phase separation in these systems. The effect of the small perturbation of the CoO₃ sublattice with either iron ions or oxygen vacancies on the bulk magnetization as well as on the M?ssbauer spectra is in good agreement with the double exchange based cluster model. The magnetoresistance does not show any peak near the Curie temperature, but reaches -84% in a field of 7.5 T at T = 8 K. Below T_C ≈ 180 K the M?ssbauer spectra distinctly include the contribution from paramagnetic and ferromagnetic regions, providing direct evidence for phase separation. No contribution to the spectra from Fe⁴⁺ ions can be observed, which is an unambiguous evidence that at low concentration iron (either directly doped or formed from ⁵⁷Co by nuclear decay) is accommodated in the cobaltate lattice as Fe³⁺ ion.     

氧气氛低温退火Pt/Ba0.8Sr0.2TiO3Pt引起的低频介电弛豫效应

研究了在不同温度区间氧气氛和氮气氛退火后处理对PtBa0.8Sr0.2TiO3Pt介电特性的影响.经过高温550℃氮气退火处理后,再放入350℃的氧气中退火,发现样品的介电特性出现了非常明显的低频弛豫现象,并且这种低频弛豫现象在350℃的氮气中退火后将会消失.通过在出现低频弛豫现象的样品的上下电极加一偏压,可以发现低频弛豫现象更加明显,并且在撤消偏压后这种增强将会逐渐减弱,直至最终恢复到偏压前的弛豫状态 关键词： 脉冲激光沉积 介电弛豫 动态随机存储器     

Preparation and characterization of La0.8Sr0.2MnO3 thin films by an excimer laser MOD process for a bolometer

La_0.8Sr_0.2MnO₃ (LSMO) films were prepared on LaAlO₃ substrates by excimer laser metal organic deposition (ELMOD) at 500 °C. The temperature dependence of resistance of the LSMO films was investigated by changing the laser fluence, irradiation time, and film thickness. It was found that the resistance of the LSMO films 80 nm in thickness that were irradiated by an ArF laser at a fluence of 100 mJ/cm² for 60 min showed a metallic temperature dependence, and the maximum temperature coefficient of resistance of the films (defined as 1/R×dR/dT) was 3.4% at 265 K. PACS 81.15.-z; 81.15.Fg; 81.15.Np; 73.61.-r; 71.30.+h     

Ba0.8Sr0.2TiO3 films crystallized on glass and platinized substrates by laser-assisted annealing at room temperature

In this work, Ba_0.8Sr_0.2TiO₃ (BST) films were grown by pulse laser ablation on bare glass and platinized substrates. The crystalline phase was obtained with the help of laser-assisted annealing (LAA) at room temperature, in air environment. By adjusting LAA conditions, like frequency of the laser and number of shots, we were able to grow crack-free BST thin films with pure perovskite phase on bare glass and platinized substrates. The crystalline layer was found to be the same irrespective of the substrate used, c.a. 250 nm thick. The electric characteristics of the amorphous and LAA crystalline BST films deposited on platinized substrate were further studied and analyzed. While in amorphous films it was found that the oxygen defects are responsible for conduction, in LAA films the amorphous/crystalline interface layer plays an important role in current leakage.     

Electrochemical oxidation and reduction of the La0.2Sr0.8CoO3−δ phases: Control of itinerant ferromagnetism and magnetoresistance

In order to understand how the magnetotransport properties are related to the oxygen content, the La_0.2Sr_0.8CoO_3−δ perovskite phases have been studied by using electrochemical cycling. The samples, prepared by a sol–gel technique, have been characterized from a structural and chemical point of view concomitantly to their physical properties. Upon reduction, a brownmillerite-type phase La_0.2Sr_0.8CoO_2.65, which presents a weak ferromagnet behavior, is formed and, upon oxidation, a complete stoichiometric phase with a cubic unit cell (a = 3.83Å) could be obtained with a ferromagnetic and metallic behavior. Finally, it is shown that using appropriate cut-off potential upon reduction or oxidation the oxygen content and the cobalt valence can be monitored in such transition metal oxides to reach the fully oxidized or reduced end-members.     

Rate limiting steps of the porous La0.6Sr0.4Co0.8Fe0.2O3−δ electrode material

The electrode reaction of porous La_0.6Sr_0.4Co_0.8Fe_0.2O_3?δ films deposited onto Ce_0.9Gd_0.1O_1.95 (CGO) was investigated by impedance spectroscopy within the temperature and oxygen partial pressure (pO₂) ranges of 500 ≤ T ≤ 700 °C and 10^? 4 < pO₂ < 1 atm, respectively, using Ar and He as gas carriers. The electrochemical impedance spectroscopy (EIS) measurements reveal a high frequency (HF) and a low frequency (LF) regions in the Nyquist plane. The high frequency (HF) region was fitted with a Warburg-type impedance element, and the low frequency (LF) region was reproduced with a resistance in parallel to a constant phase element. Both, the slight dependence of the polarization resistance (R_W) and the small variation of the apex frequency (f_v) of the HF Warburg-type element, on pO₂, suggest that this contribution corresponds to the oxygen diffusion in the bulk of the La_0.6Sr_0.4Co_0.8Fe_0.2O_3?δ electrode material. The variation of the polarization resistance of the LF region (R_rcpe) with pO₂ indicates that as T increases, the limiting step evolves from dissociative oxygen adsorption to oxygen gas diffusion in the pores of the mixed ionic/electronic conductor (MIEC) electrode.     

Synthesis and characterization of La0.9Sr0.1Ga0.8Mg0.2O3-δ electrolyte for intermediate temperature solid oxide fuel cells (ITSOFC)

Experimental investigations on new materials for application as electrolyte in electrolyte supported planar Intermediate Temperature Solid Oxide Fuel Cells (ITSOFC) operating below 800 °C is in progress at our laboratory. Sr and Mg doped Lanthanum gallate (LSGM) powder was prepared by glycine — nitrate combustion method. The prepared LSGM powder is relatively finer than that prepared through other techniques such as solid-state reaction. The measurements comprising XRD, particle size, density, TGA/DTA were made. Thin sections of circular pellets were fabricated and annealed at different temperatures ranging between 1000 and 1300 °C. The sintering behaviour of LSGM was investigated to obtain information on the densification factor, relative percentage shrinkage/expansion in volume, while annealing and the resulting apparent porosity values. Bismuth oxide is found to be an effective sintering aid in general. So the effect of bismuth oxide addition on LSGM was investigated through sintering studies, XRD, TGA/DTA, SEM and conductivity measurements. The results obtained on LSGM with and without bismuth oxide addition are discussed with respect to the requirement of an electrolyte for ITSOFC applications. Paper presented at the 2nd International Conference on Ionic Devices, Anna University, Chennai, India, Nov. 28–30, 2003.     

Commensurate water monolayer at the RuO2(110)/water interface

We demonstrate the presence of two types of commensurate, registered water monolayers with different densities at the RuO2(110)/bulk-water (0.1 M NaOH solution) interface with off-specular, oxygen crystal truncation rods. At anodic potentials (close to oxygen evolution), the extraneous water layer and the surface hydroxide layer form a bilayer with O-H-O bond distances similar to that of ice X. At cathodic potentials, the water molecules converted from the bridging OH molecules form a low-density water layer.     

Electrical conductivity and thermal expansion of La0.8Sr0.2(Mn,Fe,Co)O3-δ perovskites

La_1−xSr_xMeO₃ (Me = Mn, Co, Fe) perovskites are used as cathodes and are also attractive materials for application as the contact layer between cathode and interconnect in solid oxide fuel cells. In this contribution, three perovskite series, La_0.8Sr_0.2Mn_1−xCo_xO_3-δ (series 1), La_0.8Sr_0.2Fe_1−xCo_xO_3-δ (series 2) and La_0.8Sr_0.2Mn_1−x/2Fe_(1−x)/2Co_xO_3-δ (series 3) with x = 0, 0.25, 0.5, 0.75 and 1 were re-investigated under identical synthesis and measurement conditions with the aim of obtaining a full overview of the quasi-ternary system La_0.8Sr_0.2MnO_3-δ–La_0.8Sr_0.2FeO_3-δ–La_0.8Sr_0.2CoO_3-δ. The distribution of the different crystallographic phases in the selected series, the DC electrical conductivity and the thermal expansion coefficients are presented.     

Oxygen permeation properties and the stability of La0.6Sr0.4Fe0.8Co0.2O3 studied by Raman spectroscopy

A partial oxidation test was performed using a La_0.6Sr_0.4Fe_0.8Co_0.2O₃ (LSFCO6482) membrane in the surface limited conditions at a temperature range of 800–920 °C and in a syngas side atmosphere of CH₄ diluted with Ar (100%, 25.6% and 1%) in order to investigate chemical stability and oxygen permeation properties. On the surface, deposited carbon was observed in the test and it was revealed to be graphite by an in-situ Raman measurement. It was combusted by oxygen fed through the membrane. Segregation of the membrane on both surfaces was observed in all specimens after the test. Moreover, significant decomposition occurred on Air side surface of the membrane which tested in a syngas side atmosphere of 25.6% and 100%. However, the significant decomposition did not affect to be oxygen permeation properties, but it did affect its mechanical strength.     

Gd3+对Ca0.8Zn0.2TiO3:Pr3+红色发光材料的敏化作用

采用溶胶-凝胶法在较低温度下合成了红色发光材料Ca0.8Zn0.2TiO3:0.002Pr3 ,xGd3 .金属离子预先溶解含有柠檬酸的乙二醇溶液中.XRD表明在800℃灼烧2h形成了钛酸盐的晶体.荧光检测结果表明所合成样品的特征激发峰为330nm、特征发射峰为614nm,分别对应于O(2p)→Ti(3d)带间跃迁和Pr3 的1D2→3H4的特征发射.样品的发光强度随x的不同而不同,发光强度增强了29%-69%.当x＝0.0005时,即nPr3 :nGd3 ＝4∶1时样品的红色发光强度最强,发光强度增强了69%.     

Reactivity and diffusion between La0.8Sr0.2MnO3 and ZrO2 at interfaces in SOFC cores by TEM analyses on FIB samples

In this paper, we investigate the diffusion and reactivity at the interface between the electrolyte (YSZ) and the cathode (LSM) of planar SOFC single cells by Analytical and High Resolution Transmission Microscopy. Cells were obtained via an aqueous tape-casting process allowing casting the three layers (cathode, electrolyte, anode [Ni-YSZ]) in a single operation. To allow the cell to function at intermediate temperatures (750–850 °C), the final electrolyte thickness after co-sintering at 1350 °C must range between 40 μm and 50 μm. As-sintered cells as well as cells that have been operated at 850 °C have been considered here; it is shown that the electrical performances were one order of magnitude less than expected and that they deteriorated quickly under operating conditions.In order to explain this behavior, we carried out analytical transmission electron microcopy. To obtain precisely located chemical and structural information, we used the “H-Shape” as well as the Lift Out FIB (Focused Ion Beam) techniques to extract 5 × 10 × 0.1 μm TEM samples; additional High Resolution characterization was carried out at interfaces between LSM and YSZ grains on standard ion-milled samples. We showed that the co-sintering temperature (1350 °C) was responsible for some diffusion of manganese through the electrolyte and the cathode, leading then to the rise of a significant electronic conduction and to the drop off of the ionic conductivity, and accounts for the germination and growth of the resistive pyrochlore phase La₂Zr₂O₇. Operating the cell at 850 °C do not aggravate these phenomena, but rather alters the anode microstructure.     

Rise Kinetics of Up-Conversion Luminescence of the LiY0.8Yb0.2F4:Tm3+ (0.2 at %) Crystal with Pulsed Excitation

Physics of the Solid State - The regression analysis of the rise kinetics of up-conversion luminescence of the LiY0.8Yb0.2F4:Tm3+ (0.2 at %) crystal is performed. The kinetics curve is obtained...