This article focuses on perovskite materials for application as cathode material in solid oxide fuel cells. In order to develop new promising materials it is helpful to classify already known perovskite materials according to their properties and to identify certain tendencies. Thereby, composition-dependent structural data and materials properties are considered. Structural data under consideration are the Goldschmidt tolerance factor, which describes the stability of perovskites with respect to other structures, and the critical radius and lattice free volume, which are used as geometrical measures of ionic conductivity. These calculations are based on the ionic radii of the constituent ions and their applicability is discussed. A potential map of perovskites as a tool to classify simple ABO 3 perovskite materials according to their electrical conduction behavior is critically reviewed as a structured approach to the search for new cathode materials based on more complex perovskites with A and/or B-site substitutions. This article also covers the approaches used to influence electronic and the ionic conductivity. The advantage of mixed ionic electronic conductors in terms of the oxygen exchange reaction is addressed and their important properties, namely the oxygen-exchange coefficient and the oxygen diffusion coefficient, and their effect on the oxygen reduction reaction are presented.

The determination of ammonia volatilization with sufficient spatial and temporal resolution requires a simple and versatile in situ measurement technique, particularly in developing countries. Therefore, a simple chamber method for determining ammonia (NH 3 ) volatilization in the field (Dra¨ger-Tube Method; DTM) was calibrated by comparison with simultaneous measurements with a micrometeorological Integrated Horizontal Flux (IHF) method. Five field experiments were conducted following urea fertilization on summer maize and winter wheat plots (1998 -1999) at Fengqiu Experimental Station, Central China. The simplicity of the chamber method allowed for measurements to be carried out by trained farmers. The measurements with both methods yielded very similar patterns of NH 3 fluxes and similar differences between fertilization treatments. Cumulative NH 3 losses determined by the IHF method ranged from 14.6 to 47.9% and from 0.6 to 17.9% of urea-N applied for surface broadcast and incorporated fertilization, respectively. As expected, cumulated NH 3 losses were underestimated by the DTM as compared to the IHF by about one order of magnitude. A calibration equation was calculated by multiple linear regression which included NH 3 flux data as well as temperature and wind speed values. The calibration model yielded a modelling efficiency c 2 of 0.86 resulting in an average estimation error of cumulative NH 3 losses of 17%. The equation was validated by comparison of IHF measurements and DTM fluxes not considered in the derivation of the calibration formula. The calibration approach can be used under similar meteorological and field conditions irrespective of the soil characteristics or type of N fertilizer applied.

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Downward displacement of cadmium (Cd) and zinc (Zn) at the field scale was investigated in a sandy soil irrigated under controlled conditions for 29 yr with a total of 12.8 m3 m−2 of municipal wastewater. On a 0.86 ha grid a total of 720 samples was taken from 0 to 1.2 m depth. Cadmium and Zn contents were determined in all samples by extraction with 0.025 M ethylene‐diamine‐tetra‐acetic acid (EDTA) on the assumption that this fraction represents the solution and sorbed phases in soil. Solution phase concentrations of 480 samples were estimated by equilibrating the samples with 0.0025 M CaCl2. Heavy metal downward displacement is spatially highly variable. Five percent of EDTA‐extractable Cd and Zn were found below 0.7 and 0.9 m depth, respectively. Spatial variability of heavy metal load as measured by recovered mass per unit area reflects the geometry of irrigation systems employed. Measured sorption equilibria could be well described as a function of organic C content and pH by extending the Freundlich equation.