Solid‐oxide fuel cells based on doped ceria electrolytes and operating at 500°C are shown to be feasible. The operating regime of doped ceria electrolytes is discussed. It is shown that the ionic conductivity of ceria‐based fuel cells is sufficiently high for operation with hydrogen fuel at low temperatures. The major challenges of fabricating a thin electrolyte by a conventional method and the development of high‐performance cathodes capable of operating at 500–600°C are addressed. Cells based on thin‐film ceria electrolytes also exhibited good open‐circuit voltages between 0.97 and 1 V. Cathode materials with high performance have been developed from pyrochlores, perovskites, and cermets of silver and doped bismuth oxide. The advantages and disadvantages of different cathode materials are discussed. The maximum power density obtained at 500°C was 140 normalmW/cm2 . © 1999 The Electrochemical Society. All rights reserved.

Sequestration of carbon dioxide by steelmaking slag was studied in an atmospheric three-phase system containing industrial slag particles, water, and CO 2 gas. Batch-type reactors were used to measure the rate of aqueous alkaline leaching and slag particle carbonization independently. Four sizes of slag particles were tested for the Ca leaching rate in deionized water at a constant 7.5 pH in an argon atmosphere and for carbonate conversion with CO 2 bubbled through an aqueous suspension. Conversion data (fraction of Ca leached or converted to carbonate) were evaluated to determine the rate-limiting step based on the shrinking core model. For Ca leaching, the chemical reaction is the controlling mechanism during the initial period of time, which then switches to diffusion through the developed porous layer as the rate-limiting step. Carbonate conversion proceeded much slower than leaching conversion and was found to be limited by diffusion through the product calcium carbonate layer. The calculated value of diffusivity was found to be 5 · 10 -9 cm 2 /s, which decreased by an order of magnitude with increasing carbonization conversion as a result of changing density of the product layer. The experimental data fit the shrinking core model well after correction for the particle specific surface area.

Crystallographic structures of spheroidal graphite particles (graphite nodules) were examined using transmission electron microscopy (TEM). Structures of graphite nodules were investigated relative to different stages of nodule growth in ductile iron. Curved graphene layers were observed during the early growth of the graphite nodules. Thin layered stacking faults give rise to streaking in the basal reflections, which give rise to curvature of the nodule and growth steps on the surface. Columnar substructures consisting of parallel peripheral subgrains were found in the outside region of graphite nodules that were formed during the intermediate growth stage. Straight planar graphene layers were observed in the columnar substructures. Crystallographic orientation of graphite showed little change through the subgrain and the c-axes of multiple subgrains in a single columnar substructure were parallel. A method for characterizing the crystal structures of graphite based on the selected area diffraction pattern was introduced. Both hexagonal structure and rhombohedral structure were found in the spheroidal graphite particles. Possible crystallographic defects associated with hexagonal-rhombohedral structure transition were discussed. 1. INTRODUCTION The authors have previously reported on quenching experiments where an inoculated liquid ductile iron was examined at various stages of solidification. [1-3]. It was shown that the size of a graphite nodule reflects its growth stage [1-3]. A graphite nodule with a diameter less than ten microns is in contact with liquid phase and regarded as being retained at an early growth stage [1-2]. Smooth surfaces with circumferentially grown steps were observed in the nodules at early growth stages, and an example of a graphite nodule (seven microns diameter) at early growth stage is shown in Fig 1(a). A growth step observed on the surface of the graphite nodule is highlighted by arrow in Fig. 1(a).

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