Thermodynamic assessment of the Al-Ni system

This work is focused on the comparative analysis of electrochemical and transport properties in the major families of cathode and anode compositions for intermediate-temperature solid oxide fuel cells (SOFCs) and materials science-related factors affecting electrode performance. The first part presents a brief overview of the electrochemical and chemical reactions in SOFCs, specific rate-determining steps of the electrode processes, solid oxide electrolyte ceramics, and effects of partial oxygen ionic and electronic conductivities in the SOFC components. The aspects associated with materials compatibility, thermal expansion, stability, and electrocatalytic behavior are also briefly discussed. Primary attention is centered on the experimental data and approaches reported during the last 10–15 years, reflecting the main challenges in the field of materials development for the ceramic fuel cells.

Electrode materials and reaction mechanisms in solid oxide fuel cells: a brief review

Ferrite-based perovskites as cathode materials for anode-supported solid oxide fuel cells. Part I. Variation of composition

A short overview is given for the thermal expansion of solid oxide fuel cell materials The thermomechanical compatibility of state-of-the-art materials is compared with alternative, new materials With these alternatives a better adjustment of the thermal expansion coefficients of the various fuel cell components is possible and fuel cells based on the newly developed materials are proposed

Thermal expansion of SOFC materials

The paper critically reviews the state of the art on the oxidation of TiAl. At the beginning differences compared to the oxidation of conventional Fe, Ni and Co alloys are discussed. The Knowledge of the TiAlO phase diagram as a basis for an understanding of the processes occurring during oxidation is very incomplete. Three different scale structures can be distinguished as functions of time or scale thickness. In the sub-surface zone two phases with high oxygen solubilities can be identified after a certain time. These phases do not follow from the existing phase diagram. Nitrogen can have two different effects depending on the time it is added to the oxidation environment. The micro-structure also affects the oxidation behaviour significantly, so this effect can lead to a misinterpretation of the influence of other parameters such as alloying elements. The influence of alloying elements is not yet understood. Mechanical loading leads to scale cracking, however, the system has a significant scale repair capacity, though about twothirds of the scale grows by inwards transport.



Grundlagen der TiAl-Oxidation – Eine kritische Ubersicht



Die Arbeit gibt eine kritische Betrachtung der Kenntnisse uber die Oxidation von TiAl. Einleitend wird auf Unterschiede im Vergleich zur Oxidation konventioneller Fe. Ni und Co-Legierungen hingewiesen. Die derzeitigen Kenntnisse uber das Phasendiagramm TiAlO als Basis fur das Verstandnis der bei der Oxidation ablaufenden Prozesse sind sehr unvollstandig. Im Aufbau der Oxidschicht konnen drei Grenzfalle unterschieden werden, die als Funktion der Zit nacheinander durchlaufen werden. Stickstoff kann offensichtlich zwei Effekte auslosen, abhangig vom Zeitpunkt der Zugabe zum Reaktionsgas. Die Mikrostruktur kann einen starken Einflus haben und andere Einflusse wie die von Legierungselementen verdecken. Der Einflus von Legierungselementen wird derzeit nicht verstanden. Damit sind auch keine sinnvollen Vorhersagen moglich. Mechanische Belastungen verursachen ein Aufreisen der Oxidschicht, die jedoch eine beachtliche Ausheilfahigkeit fur Oxidschichtrisse hat.

Fundamentals of TiAl oxidation: a critical review

Chemical compatibility of the LaFeO3 base perovskites (La0.6Sr0.4)zFe0.8M0.2O3 − δ (z = 1, 0.9; M = Cr, Mn, Co, Ni) with yttria stabilized zirconia

The Al-Ni phase diagram has been investigated in the com position range xNi = 0.70 to 0.97. Phase boundaries were determined by using differential thermal analysis and Knudsen effusion mass spectrometry. The measurements were carried out in the temperature range between 1409 and 1730K. An Al-Ni phase diagram is obtained for xNi ≧ 0.70 by combining the data from this work with selected data from the literature. This diagram deviates from that recommended by phase diagram compilations and used generally in the literature to date; it agrees reasonably well with a diagram which has been rejected in the literature.

Phase Diagram Studies on the Al- Ni System

This paper reports on strontium-doped lanthanum manganite powders that were prepared using a peroxide acetate salt based solution. The stable sol was peptized by reacting ammonium hydroxide with the precursor solution. The amorphous dried gel powders exhibit a high energy level, due to their high cations coordination and small particles, to develop the perovskite phase. This crystalline phase development from powders containing monocarboxylate ligands was characterized by thermal analysis (TG, DTG, DTA), X-ray diffraction, and IR spectroscopy. The transformation from amorphous powders into a crystallized homogeneous oxycarbonate phase in a first stage corresponds to an exothermal DTA peak at 270{degrees} C. X-ray diffraction patterns and IR spectra showed similar behavior of the powders after complete organic removal, during the conversion into perovskite phase starting at approximately 630{degrees} C and achieved about 700{degrees} C, as well as during the sintering process.

Sol—Gel Synthesis of Strontium‐Doped Lanthanum Manganite

Orthoferrite-based perovskites are of interest as materials for the cathode in solid oxide fuel cells (SOFCs). Therefore, the chemical compatibility between perovskites of the composition (La1−xSrx)zFe1−yMnyO3−δ (0 # x # 0.3; 0.2 # y # 1; z = 0.90, 0.95, 1.00) and the solid electrolyte zirconia (ZrO2) doped with 8 mol% yttria (Y2O3) (8YSZ) has been investigated. Powder mixtures of the two materials have been annealed at different temperatures. The formation of monoclinic ZrO2 at 1000°C, as well as of La2Zr2O7 and SrZrO3 at 1400°C, has been determined in some samples. The reactions that are observed are discussed, with respect to the thermodynamic activities, tolerance factor, and oxygen-ion migration energies. Some perovskite compositions seem to be compatible with Y2O3-stabilized ZrO2 (YSZ), thereby offering the possibility to use orthoferrite-based perovskites in SOFCs with a solid electrolyte made of YSZ.

Chemical Interactions between La-Sr-Mn-Fe-O-Based Perovskites and Yttria-Stabilized Zirconia

The vaporization of liquid and solid alloys of the Al — Ni system in the range xNi = 0.7 to 1 has been extensively investigated by the use of Knudsen effusion mass spectrometry in the temperature range between 1389 and 1734 K. 15 alloy samples of different compositions of the complete concentration range were studied in the course of 25 runs and their Al and Ni partial pressures determined. The vapour pressure measurements in the course of the individual runs were carried out over temperature ranges of up to about 300 K covering the solid and the liquid state for many of the alloys investigated. Chemical activities and chemical potentials for Al and Ni as well as Gibbs energies were evaluated at 1600 K for the solid and at 1728 K for the liquid state. Two independent methods were used for the determination of Ni activities of the solid alloys yielding consistent results. The large temperature range of the partial pressure measurement for the solid AlNi3 phase with xNi = 0.750 rendered possible the determination of a complete set of partial and integral thermodynamic functions (ΔG, ΔH, ΔS) for the formation of this phase. The values obtained are discussed. The partial and integral enthalpies of formation are compared with the enthalpies of dissociation of the gaseous species Ni2(g), Al2(g), and AlNi(g). The integral enthalpy of formation, obtained for the first time by vapour pressure measurements, agrees well with the results of a recent calorimetric study.

Hubertus Nickel

Papers

Chemical compatibility of the LaFeO3 base perovskites (La0.6Sr0.4)zFe0.8M0.2O3 − δ (z = 1, 0.9; M = Cr, Mn, Co, Ni) with yttria stabilized zirconia

Phase Diagram Studies on the Al- Ni System

Sol—Gel Synthesis of Strontium‐Doped Lanthanum Manganite

Chemical Interactions between La-Sr-Mn-Fe-O-Based Perovskites and Yttria-Stabilized Zirconia

Thermodynamic Study of the Liquid and Solid Alloys of the Nickel-Rich Part of the Al — Ni Phase Diagram Including the AlNi3 Phase