Thermal perovskite

Calcination. Calcination involves a low (<1000° C) temperature soHd-state chemical reaction of the raw materials to form the desired final composition and stmcture such as perovskite for BaTiO and PZT. It can be carried out by placing the mixed powders in cmcibles in a batch or continuous kiln. A rotary kiln also can be used for this purpose to process continuously. A sufficiendy uniform temperature has to be provided for the mixed oxides, because the thermal conductivity of powdered materials is always low. 

A signihcant problem in tire combination of solid electrolytes with oxide electrodes arises from the difference in thermal expansion coefficients of the materials, leading to rupture of tire electrode/electrolyte interface when the fuel cell is, inevitably, subject to temperature cycles. Insufficient experimental data are available for most of tire elecuolytes and the perovskites as a function of temperature and oxygen partial pressure, which determines the stoichiometty of the perovskites, to make a quantitative assessment at the present time, and mostly decisions must be made from direct experiment. However, Steele (loc. cit.) observes that tire electrode Lao.eSro.rCoo.aFeo.sOs-j functions well in combination widr a ceria-gadolinia electrolyte since botlr have closely similar thermal expansion coefficients. 

Recent applications of e-beam and HF-plasma SNMS have been published in the following areas aerosol particles [3.77], X-ray mirrors [3.78, 3.79], ceramics and hard coatings [3.80-3.84], glasses [3.85], interface reactions [3.86], ion implantations [3.87], molecular beam epitaxy (MBE) layers [3.88], multilayer systems [3.89], ohmic contacts [3.90], organic additives [3.91], perovskite-type and superconducting layers [3.92], steel [3.93, 3.94], surface deposition [3.95], sub-surface diffusion [3.96], sensors [3.97-3.99], soil [3.100], and thermal barrier coatings [3.101]. 

Surface reconstruction has been earlier observed and reported in the literature [116]. Sequential reductive and oxidative thermal treatment usually leads to bulk transition from CoOx + La203 to LaCo03, respectively. On the other hand, the restoration of the perovskite structure is not observed under severe conditions at higher temperature. In those temperature conditions, the sintering of Co crystallites leads to irreversible redox cycle with the preferential formation of Co304 under lean conditions. 

In this chapter the technological development in cathode materials, particularly the advances being made in the material s composition, fabrication, microstructure optimization, electrocatalytic activity, and stability of perovskite-based cathodes will be reviewed. The emphasis will be on the defect structure, conductivity, thermal expansion coefficient, and electrocatalytic activity of the extensively studied man-ganite-, cobaltite-, and ferrite-based perovskites. Alterative mixed ionic and electronic conducting perovskite-related oxides are discussed in relation to their potential application as cathodes for ITSOFCs. The interfacial reaction and compatibility of the perovskite-based cathode materials with electrolyte and metallic interconnect is also examined. Finally the degradation and performance stability of cathodes under SOFC operating conditions are described. 

An alternative to the Co-rich perovskites is the Sr-doped LaFe03 which has a lower thermal expansion coefficient and a superior chemical compatibility with doped Ce02 electrolyte. LaFe03 is expected to be more stable than Ni- and Co-based perovskites because the Fe3+ ion has the stable electronic configuration 3d5. It is, therefore, expected that compositions in the system (La,Sr)(Co,Fe)03 will have desirable properties for intermediate temperature SOFC cathode applications. 

The Sr-doped praseodymium manganites and cobaltites have been studied by several groups as potential cathodes for ITSOFC. Kostogloudis et al. [116-118] systematically investigated the crystal structure, electrical conductivity, and thermal expansion properties of (Pr, Sr)Mn03, (Pr, Sr)Co03, and (Pr, Sr)(Co, Mn)03 systems. All compounds have the orthorhombic perovskite GdFe03-type structure (Pbnm... 

Hydrocracking, 30 48-52 behavior, thermal, 29 269 catalytic, 26 383 deethylation, 30 50 demethylation, 30 50 metallocarbene formation, 30 51-52 of f -decane, 35 332-333 primary coal liquids, 40 57 procedure, 40 66-67 product distribution, 30 49 reactions, over perovskites, 36 311 suppression by sulfur, 31 229 zeolite-supported catalysts, 39 181-188... 

Evolution of secondary phases. Another concern has been continued formation of LZ and SZ secondary phases at the perovskite/YSZ interface as a function of time or current density. - Accelerated testing, achieved by sustained heat treatments of the electrode, suggests that degradation can occur by this mechanism.However, whether such thermal treatments can be meaningfully extrapolated to predict natural degradation processes is unclear. 

Co-, Mn-, and Fe-based perovskites, NO peaks appearing at similar temperatures as those in NO-TPD experiments (apart from a minor desorption at medium temperature and an intense one at high temperature) were also found and aseribed to the desorptions of physically adsorbed NO, nitrosyl species and nitrate species in the order of their thermal stability. The NO peaks obtained at low (80-110 °C), medium (200-210 °C), and high (310-390 °C) temperatures in the present NO-TPD analyses were thus correlated to physically adsorbed NO, nitrosyl and nitrate species. 

Qi et al. studied perovskite systems obtained by partially replacing La with Ce at the A-site of LaNi03 for ATR of gasoline and its subrogate n-octane with or without thiophene additive. They found similar activities and mechanical and thermal stabilities with both pellet and monolith Lao.8Ceo.2Ni03 [73]. 

---