François Morin

The phase stability of La0.5Sr0.5CoO3 − δ

Abstract In preparing ABO 3 compounds with A = LaandSrandB = CoorMn, it has been found that strontium-substituted lanthanum cobaltite, La 1 − xSr xCoO 3 − δ, has much less tolerance for any variation in the A/B ratio than its manganite counterpart. This is specifically demonstrated for La 0.5Sr 0.5CoO 3 − δ where distinct phases readily appear on either sides of the A/B ratio for any small departure of this ratio from unity. These secondary phases are clearly evidenced by scanning electron microscopy and X-ray diffraction. The same observations have been extended to various degrees of strontium substitution between 0.4 ≤ x ≤ 0.6. Additional data in regard to the main phase stability as a function of the oxygen partial pressure at temperatures up to 1425 °C have also been gathered. The (La 1 − ySr y) 2CoO 4-phase formed in this latter case is very similar to that due to the unbalance in the A/B ratio.

Pulsed laser deposition of superionic ceramic thin films: deposition and applications in electrochemistry

Pulsed laser deposition (PLD) has been used to prepare two types of superionic ceramic thin films: (i) NASICON (Na superionic conductor) for a Na ion selective membrane and (ii) the mixed electronic and oxygen conductors La1−xSrxMnO3±δ (LSM) for the cathode in solid oxide fuel cells. NASICON thin films have good ionic conductivity with resistivities between 3×103 and 5×104 Ω cm at room temperature and they have low surface particle densities depending upon the deposition conditions. When NASICON is deposited on polymeric (Raipore) membranes frequently employed for Na selectivity, the composite membranes show a significant increase of current efficiency of 23% for the production of sodium hydroxide from sodium sulfate using electrolysis. For LSM, very smooth and dense thin films are produced by PLD with the perovskite structure after an anneal at 950°C.

The growth of oxide platelets on nickel in pure oxygen. I. Morphology and oxidation kinetics

A very specific oxide morphology consisting in rounded oxide platelets a few microns in diameter has been obtained on pure nickel. The main experimental factors in the achievement of such a controlled morphology have been thoroughly investigated. Platelets are produced in the temperature range from 600 to 850°C and for oxidation times varying from a hundred hours to a very few minutes. Platelets could not be grown on very pure nickel. Scanning electron microscopy has been used extensively, and the corresponding observations are reported in the first part of this study together with thermogravimetric results.

The growth of oxide platelets on nickel in pure oxygen. II. Surface analyses and growth mechanism

The structural properties of NiO platelets emerging from a primary oxide layer by oxidation of pretreated nickels in pure oxygen between 650 and 800° C have been investigated in relation with the initial metallic layers and the primary oxide. Surface composition and segregation of impurities were also studied by X-ray photoelectron spectroscopy and Auger electron spectroscopy. Textural properties and structural orientation of both the primary oxide layer and the platelets were analyzed by X-ray diffraction and transmission electron microscopy. Platelets grew along {111} planes, leading to elliptical or semicircular bicrystals. The driving force for the present type of growth originates from the stresses accumulated in the thin, primary oxide layer. Impurities do not directly interact with this growth, but enable a specific grain structure to be developed thereby favoring platelet growth.

True chemical diffusivity and surface reactivity of cobaltous oxide

Abstract Chemical relaxation experiments in cobaltous oxide at 1000°C have been performed in order to reassess previous observations of a variation of the apparent diffusivity vs the oxygen partial pressure in the range of 10 −4 -1 atm. By making use of significantly different crystal sizes and by studying the initial reequilibration process in more detail, a slow surface reaction kinetics has been newly evidenced for cobaltous oxide, even in pure oxygen close to 1 atm. This slow surface reaction has been found to be responsible for variations of the apparent diffusivity previously reported. In the true diffusional regime, the chemical diffusion coefficient ~ D is equal to about 9.1–9.2 · 10 −1 cm 2 s −1 which is very close to the highest value of ~ D previously reported by the same authors.

A reassessment of chemical diffusivity in TiO2−x

Abstract Chemical diffusivity has been measured in rutile single crystals, at 1100°C and perpendicularly to the c-axis. Reequilibration kinetics are considerably slowed down by a surface reaction in CO/CO 2 mixtures while this effect vanishes in pure oxygen atmospheres. The chemical diffusion coefficient D was found to be nearly independent of x in TiO 2−x .

Modeling of complex point defects in transition metal compounds

Abstract The nature and the concentration of point defects in transition metal compounds is usually inferred from nonstoichiometry and electrical conductivity data in conjunction with the assumption of an ideal defect solution and constant electronic mobility. The extent to which this approach can be used to unambiguously confirm the existence of minority defects is investigated here. Acceptability criteria are suggested and an analytical treatment is specifically given. Examples are provided for p-type CoO and n-type TiO2.

An X-ray powder diffraction study of lanthanum–strontium ferromanganites

Abstract Various lanthanum–strontium ferromanganites (La 1− x Sr x )(Mn 1− y Fe y )O 3± δ (LSMF) with x =0.2, 0.5 and 0.7 and y =0.2, 0.5 and 0.8 were prepared by a glycine–nitrate combustion route and conditioned into two different oxygenation states which are likely to be encountered in cathode materials for solid oxide fuel cells (SOFC). Crystal symmetries for both the low and the fully oxygenated states were determined by X-ray diffractometry. Most perovskite compositions crystallized in either cubic or rhombohedral symmetry with the exception of La 0.5 Sr 0.5 Mn 0.2 Fe 0.8 O 3± δ , where a transition from rhombohedral to cubic occurred in going from the full to the low oxygenation state. In addition, two strontium-poor compositions, La 0.8 Sr 0.2 Mn 0.5 Fe 0.5 O 3± δ and La 0.8 Sr 0.2 Mn 0.2 Fe 0.8 O 3± δ , remained diphasic and consisted of a combination of both the rhombohedral and the orthorhombic phases. Changes in unit cell volume were analyzed as a function of chemical composition and oxygenated state.

High-temperature surface reactivity of TiO2 in CO/CO2 mixtures: A kinetic study

Abstract The surface reactivity of rutile single crystals in CO/CO 2 mixtures, at 1100°C, has been followed by means of a re-equilibrium technique and electrical conductivity measurements. Kinetics results are complemented by additional information from surface analysis techniques (AES, RHEED and SEM). In contradistinction with pure oxygen, the surface reaction in CO/CO 2 is always rate-determining and a semilogarithmic law is followed rather closely. Platinum, chemically deposited on the initial surfaces, exerts a clear promoting effect although the catalytic properties of platinum may be seriously inhibited by a likely encapsulation effect.

An experimental setup for the study of gas-solid reequilibration phenomena

Relaxation of the chemical-potential gradient in reequilibration experiments with transition-metal compounds usually leads to the determination of the chemical-diffusion coefficient. An accurate determination of this parameter often requires stringent experimental conditions. The same is true of the characterization of slow surface reactions with oxidizing-reducing mixtures. The experimental setup described herein should comply with these requirements. It has been tested mainly with rutile.