Jae Sik Yoon

Corrosion resistance of austenitic stainless steel separator for molten carbonate fuel cell

STS310S and SC-STS310S (simultaneously co-deposited chromium and aluminum onto 310S austenitic stainless steel substrate by pack-cementation process) were used as separator materials on the cathode side of a molten carbonate fuel cell. With the STS310S, corrosion proceeded via three steps; a formation step of unstable corrosion product, a protection step against corrosion until breakaway, and an advance step of corrosion after breakaway. The final corrosion product was LiFeO2 and the loss of mass was 6.5 mg/cm2 after a corrosion test of 480 hr at 650°C. The SC-STS310S showed more effective corrosion resistance, however, than did common STS310S. There was especially no corrosion loss on the SC-STS310S after the 480 hr corrosion test. It is anticipated that it will be very useful as an alternative separator on the cathode side off the MCFC in the future.

Production of Tantalum Powder by External Continuous Supply of Feed Material and Reductant

In order to produce high-quality tantalum powder of spherical shape with a uniform particle size and a high purity using metalothermic reduction process, it is important to control the reaction temperature and reaction rate.In this study, we developed a metallothermic reduction method using the system supplying materials continuously from the external unit where the feed material of K2TaF7 and Na as a reductant were continuously supplied into the reaction furnace and the reaction rate was controlled.The characteristics of tantalum powder produced by the continuous supply system were compared with those of the powder produced by a batch system that the whole amount of the feed material and reductant were supplied all together into the reaction furnace at one time.We could get tantalum powder with spherical shape and uniform particlesize with the diameter of 2∼3 μm because we used the external continuous supply system where the reaction temperature and reaction rate could be well controlled.After deoxidation, dehydration, and heat treatments, the purity of the tantalum power reached to 99.5 mass% level of the reagent grade. The yield of tantalum powder increased from 80% in the batch system to 90% in the external continuous supply system

The preparation of tantalum powder using a MR-EMR combination process

In the conventional metallothermic reduction (MR) process used to obtain tantalum powder in batch-type operation, it is difficult to control the morphology and location of the tantalum deposits. In contrast, an electronically mediated reaction (EMR) process is capable of overcoming this difficulty. It has the advantage of being a continuous process, but has the disadvantage of a poor reduction yield. A process known as the MR-EMR combination process is able to overcome the shortcomings of the MR and EMR processes. In this study, an MR-EMR combination process is applied to the production of tantalum powder via sodium reduction of K2TaF7. In the MR-EMR combination process, the total charge passed through an external circuit and the average particle size (FSSS) increase as the reduction temperature increases. In addition, the proportion of fine particles (−325 mesh) decreases as the reduction temperature increasess. The tantalum yield improved from 65 to 74% as the reduction temperature increased. Taking into account the charge, impurities, morphology, particle size and yield, a reduction temperature of 1123 K was found to be optimum for the MR-EMR combination process.