Hyeoung Ho Park

Formation of bioceramic coatings containing hydroxyapatite on the titanium substrate by micro-arc oxidation coupled with electrophoretic deposition

Bioactive ceramic coatings on titanium substrates were prepared successfully by micro-arc oxidation coupled with electrophoretic deposition (MAO and EPD) in NaOH electrolyte solution containing hydroxyapatite (HA) particles. The HA suspensions with various NaOH concentrations were prepared by ultrasonic dispersion. The microstructure, as well as the elemental and phase composition of the coatings was examined by scanning electron microscopy, energy dispersive spectrometry, and X-ray diffraction. X-ray diffraction showed that the coatings were composed mainly of rutile, Na(2)Ti(6)O(13), and HA phases. The composition and surface morphologies are strongly dependent on the NaOH electrolyte concentration. The corrosion behavior of the coating layers in simulated body fluids was evaluated using a potentiodynamic polarization test. The corrosion resistance of the coated sample was increased compared with the untreated titanium sample. The in vitro bioactivity assessment showed that the MAO and EPD-treated titanium substrate possesses higher apatite-forming ability than the only MAO-treated titanium substrate. In addition, the cell behavior was also examined using cell proliferation assay, cell morphology, and alkaline phosphatase activity. They obtained an agreement with the result of apatite-forming ability. The results clearly show that combining the MAO and EPD techniques provides an optimal surface for cell differentiation and osseointegration.

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