M.M. Nasrallah

Structure and electrical properties of La1−xSrxCo1−yFeyO3. Part 1. The system La0.8Sr0.2Co1−yFeyO3

Abstract Crystal structure, thermal expansion, thermogravimetry, thermoelectricity, and electrical conductivity of compositions in the system La0.8Sr0.2Co1−yFeyO3 with 0 ≤ y ≤ 1 were studied as function of Co Fe ratio and temperature, in air. The electrical conduction mechanism is attributed to the adiabatic-hopping of p-type small polarons. At high temperatures, oxygen deficiency causes lattice expansion and a reduction in electrical conductivity. The observed temperature dependence of the Seebeck coefficient is attributed to changes in carrier concentration caused by a thermally excited charge disproportionation of Co3+ ions and by the ionic compensation of induced oxygen vacancies. The measured electrical conductivity and Seebeck coefficient as a function of the Co Fe ratio is interpreted using a two-site hopping and the site-percolation model. It is suggested that a preferential electronic compensation of Fe ions over Co ions may occur in this system.

Structure and electrical properties of La1 − xSrxCo1 − yFeyO3. Part 2. The system La1 − xSrxCo0.2Fe0.8O3

Abstract Crystal structure, thermal expansion, oxygen stoichiometry, thermoelectricity, and electrical conductivity of compositions in the system La 1 − x Sr x Co 0.2 Fe 0.8 O 3 with 0 ≤ x ≤ 0.6 were studied as function of temperature and Sr content, in air. The solubility of Sr in the sintered perovskite-type oxide (ABO 3 ) was limited to x ≤ 0.4. The observed p -type electrical conduction appeared to occur via a small-polaron hopping mechanism. The thermally-induced oxygen loss caused a lattice expansion plus decreases in both the carrier concentration and the carrier mobility. A semi-empirical model was developed which takes into account the thermally activated disproportionation of Co 3+ ions into Co 4+ and Co 2+ pairs, and the ionic compensation of oxygen vacancies formed at high temperatures. The concentrations of B-site ions (Co or Fe) in different valence states were calculated using this model and experimental data.

The effect of yttrium and thorium on the oxidation behavior of Ni-Cr-Al alloys

The effect of quaternary additions of 0.5% Y and 0.5 and 1.0% Th to a base alloy of Ni-10Cr-5Al on the oxidation behavior and mechanism was studied during oxidation in air over the range of 1000–1200°C. The presence of yttrium decreased the oxidation kinetics slightly, whereas the addition of thorium caused a slight increase. Oxide scale adherence was markedly improved by the addition of the quaternary elements. Although a number of oxides formed on yttrium-containing alloys, quantitative x-ray diffraction clearly showed that the rate-controlling step was the diffusion of oxygen through short-circuit paths in a thin layer of alumina that formed parabolically with time. Mixed oxides containing both aluminum and yttrium formed by the reaction of Y2O3 to form YAlOP3 initially, and Y3Al5O12 (YAG) after longer times. Although the scale adherence of the yttrium-containing alloy was considerably better than the base alloys, spalling did occur that was attributed to the formation of the voluminous YAG particles that grew in a “mushroom”-like manner, lifting the protective scale off the substrate locally. The YAG particles formed primarily at grain boundaries in the substrate in which the yttrium originally existed as YNi9. This intermetallic compound reacted to form Y2O3, liberating metallic nickel that subsequently reacted to form NiO or NiAl2O4 spinel or both. The Y2O3 reacted with aluminum to ultimately form the YAG “mushrooms.” Thorium did not form any mixed oxides; the only oxide involving thorium was ThO2, which existed as small particles at the oxide-metal interface. A highly beneficial effect of the thoria particles in reducing film spalling was observed. Scale spalling in the base alloy was attributed to void formation at the oxide-metal interface, the voids forming by condensation of excess vacancies from the Kirkendall effect associated with fast back-diffusion, of nickel into the substrate as aluminum was preferentially oxidized and diffused slowly outward. The mechanism of improved scale adherence in the quaternary alloys was the elimination of voids by annihilation of the Kirkendall vacancies at vacancy sinks introduced by the noncoherent interfaces between yttrium and thorium-containing intermetallics or oxides or both.

Synthesis and Characterization of ( CeO2 ) 0.8 ( SmO1.5 ) 0.2 Thin Films from Polymeric Precursors

(CeO{sub 2}){sub 0.8}(SmO{sub 1.5}){sub 0.2}(CSO) has been suggested for solid oxide fuel cell (SOFC) applications, because of its higher electrical conductivity and chemical stability, compared to other ceria-based materials. However, CSO is difficult to densify under conditions compatible with other SOFC components. A thin film synthesis technique has been developed for the fabrication of dense CSO films from precursor solutions at relatively low temperatures. Dense, smooth, and homogeneous films were obtained on Pt, single-crystal Si, and La{sub 0.6}Sr{sub 0.4}Co{sub 0.2}Fe{sub 0.8}O{sub 3} (LSCF) substrates by spin-coating a polymeric precursor and subsequent heat-treatment. Crystallization of the film occurred at temperatures as low as 320 C. The developed oxide film did not react with La{sub 0.8}Sr{sub 0.2}MnO{sub 3}, LSCF, or (ZrO{sub 2}){sub 0.84}(YO{sub .15}){sub 0.16}(YSZ) at temperatures up to 1,200 C. When a CSO layer was applied between LSCF and YSZ, a significant improvement in the interfacial resistance was observed. The results suggest that CSO can be used as a buffer layer on YSZ electrolytes for improved performance of high temperature SOFCs.

Immittance response of La{sub 0.6}Sr{sub 0.4}Co{sub 0.2}Fe{sub 0.8}O{sub 3} based electrochemical cells

The ac small-signal response of La{sub 0.6}Sr{sub 0.4}Co{sub 0.2}Fe{sub 0.8}O{sub 3} (LSCF) is examined via lumped parameter/complex plane analysis (LP/CPA) technique. The ac data, acquired in the frequency range 10{sup {minus}1} {<=} f {<=} 10{sup 6} Hz, are used to establish the role of ionic contribution in the LSCF material system using a two-terminal blocking electrode method. Spun-on yttria (8 mole percent (m/o) Y{sub 2}O{sub 3}) stabilized zirconia (8YSZ) or Ce{sub 0.85}Sm{sub 0.15}O{sub 2} (CSO) thin film coatings (0.3--1.0 {mu}m) is used as electron blocking electrodes, The temperature dependence of the contributing elements between the terminals, obtained via LP/CPA technique and dc measurements, is evaluated. The activation energy, associated with possible oxygen transport via an anion vacancy mechanism, is found to be {approximately}1.1 eV in the temperature range of 400--800 C. The effect of thickness and composition of the blocking electrodes are discussed, Finally, the advantages of the thin-film blocking electrodes compared to mechanical contact blocking electrodes are also presented.

Synthesis and characterization of YSZ thin film electrolytes

Abstract Dense, homogeneous and crack-free thin films (0.2−2 microm in thickness) of (ZrO2)0.84(YO1.5)0.16 (YSZ) were deposited on porous or dense substrates, at temperatures not exceeding 600°C, using a solution-deposition technique. The structural evolution and microstructure of the deposited films were investigated using X-ray diffraction (XRD) and scanning electron microscopy (SEM). Raman spectroscopy revealed the presence of fine-grain cubic YSZ at annealing temperatures as low as 600°C. Grain growth occurred at higher temperatures as detected by atomic force microscopy (AFM). ac impedance spectroscopy was used to study the electrical characteristics of the YSZ films as a function of temperature. The ionic conductivity and activation energy of the deposited film are similar to YSZ bulk material, but no grain-boundary effect was observed in the film. The deposited films may be considered for electrolyte application in intermediate temperature solid oxide fuel cells (SOFCs).

Oxidation-reduction behaviour of La-doped SrTiO3

Thermogravimetric measurements as a function of oxygen activity were performed in the temperature range of 1200 to 1400° C on a series of lanthanum SrTiO3 compounds. A model which assumes the absorption of excess oxygen in the structure which compensates the donors by formation of cationic defects is postulated. Comparison of this model with the experimental results show reasonable agreement.

Electrical conductivity, seebeck coefficient and defect chemistry of Ca-doped YCrO3

The electrical transport behavior and defect structure of Ca-doped YCrO3 were studied using electrical conductivity and Seebeck coefficient measurements as a function of oxygen activity and temperature. Defect models derived from the electrical conductivity data were found to adequately relate the concentration of charge carriers to the acceptor dopant and oxygen vacancy concentrations. Thermodynamic properties were calculated and found to be consistent with the model. Activation energy and carrier mobility data were also obtained. The analysis of the electrical conductivity, Seebeck and mobility data suggest that the conduction process in Ca-doped YCrO3 occurs via the small polaron hopping mechanism.

High-temperature redox behavior of doped SrTiO3 and LaCrO3

Abstract The stability of high-temperature fuel cell electrodes to their ambient environment is important for the long-term reliability of fuel cells. In this report the behavior of oxide electrode materials as a function of oxygen activity and temperature is considered. Models for the oxidation-reduction behavior of both p - and n -type oxides are presented. These models take into account the absorption and evolution of oxygen which take place as oxygen activity is varied. The resulting instability in electrical conductivity is explained as a consequence of changes in carrier concentration due to variability in ionic defect concentration. The proposed models are applied to acceptor-doped LaCrO 3 and donor-doped SrTiO 3 . It is shown that the models explain the experimental data well and as a consequence diagrams can be made which show the regions of oxygen activity and temperature for which stability of electrical conductivity and defect structure might be expected.

Oxygen activity dependence of the defect structure of La-doped BaTiO3

A model for the behaviour of donor-doped BaTiO3 as a function of oxygen activity is proposed. Thermogravimetric measurements show good agreement with the proposed model. The results show that donor-doped BaTiO3 is stoichiometric with electronic compensation of the donor at low oxygen activity, but with increasing oxygen activity, compensation becomes ionic as excess oxygen is absorbed. The possible compensation mechanisms are discussed.