Jae Chun Lee

Ultrasonically Accelerated Crystallization Processing for the Nano-Size Ferrite Powder

A new technique capable of accelerating the crystallization of nano-size ferrite powder at low temperature is developed. The effects of ultrasonic waves on the crystallization were studied for ferrite powders prepared using the co-precipitation method. The crystallization of the ferrite powders irradiated by ultrasonic waves was characterized by XRD. The ferrite powder prepared without ultrasonic irradiation by the co-precipitation method was amorphous. However, the ferrite prepared with ultrasonic irradiation for 5 h was crystallized and the crystallization of the ferrite powder became more enhanced in proportion to the time of ultrasonic irradiation. The ferrite powder prepared with ultrasonic irradiation for 25 h had a higher crystallinity and a larger specific surface area than the ferrite powder calcined at 500 °C for 2 h after preparing with co-precipitation. The prepared ferrite powders were a nano-size crystal phase.

Sealing Behavior of Visco-Elastic Composite Seals for SOFC Applications

Porous composites containing ceramic fiber have been developed for the fabrication of SOFC seals. They were fabricated using glass powder and alumino-silicate chopped fibers. Effect of mixing ratios of ceramic fiber and glass on the leak rates and strength of the composite seals was investigated. In addition, seal performance of commercial glasses was compared with that of SiO2-BaO-B2O3 glass synthesized in this work. The leak rate of the composite seals containing 55 vol% glass was seven times higher than the one containing 75 vol% glass. The flexural strength of the composite seals was reduced to one fourth of the initial value as the porosity increased from 1 to 29%. The incorporation of alumino-silicate chopped fibers into a sealing glass degraded room temperature strength and increased leak rates due to increase in porosity with increasing fiber content. The viscosity of glass at the seal test temperature is presumed to affect the leak rate of the glass seal.

Synthesis of Solution-Combusted ZnO Nanopowder and Its Application to Gold Recovery

Photocatlytic ZnO nanopowders were synthesized by a novel method referred to as “solution-combustion method (SCM)”. They were synthesized using various oxidants and fuels. Single-phase ZnO powders were obtained. The ZnO powder synthesized using Zn(OH)2 and glycine as an oxidant and a fuel at fuel/oxidant ratio of 0.8, showed best powder characteristics such as average grain size of 30 nm and the specific surface area of 120 m2/g. The photocatalytic gold recovery efficiency by this SCM ZnO nanopowders was about 6 fold higher compared to the case by the state of the art commercial TiO2 nanopowders. The purity of recovered gold was about 99.6% in weight% and 98.8% in atomic%. Especially, in this study it was tried to recover gold metal ions from not simulated wastewater but real plating one. This technology is therefore very viable and cost-effective to obtain high purity gold from plating wastewater.

Sealing Properties of Ceramic Fiber Composites for SOFC Application

For the sealing of solid oxide fuel cells (SOFCs), mid-term (~1,000 h) sealing performance was examined using composite seals prepared with mixtures of a glass and alumino-silicate ceramic fiber. Leak rate could be reduced to < ~0.04 sccm/cm when using a composite seal with 60 vol% glass. Viscosity of the glass at the seal operating temperature of 650 was 2.0×108 dPa·s (log η = 8.3) as estimated by beam-bending method, and found to be suitable for sealing operation.

Removal of Heavy Metal Ions from Aqueous Pb-EDTA and Cu-EDTA Solutions Using Nanosized ZnO Powders by Solution-Combustion Method

ZnO nanopowder was prepared by a novel “solution-combustion method (SCM)” and it was used as a semiconductor photocatalyst to evaluate its photoreduction properties. Aqueous Pb-EDTA and Cu-EDTA solutions of heavy metal ions (Pb and Cu) were used for the photocatalysis reaction under UV illumination. The result was then compared with other semiconductor photocatalyst powder such as TiO2 powder (P25; Degussa) and TiO2 powder prepared by homogeneous precipitation process at low temperature (HPPLT). In the removal of Pb++ ions, the ZnO nanopowder showed 2.6 fold higher removal rate than P25 TiO2 powder and 1.8 fold higher than HPPLT TiO2. Also the ZnO nanopowder showed the highest removal rate of the Cu++ ions from the solution among the different photocatalyst powders compared. The superior photoreduction ability of the ZnO nanopowder appears to be due to its excellent UV absorption characteristics. This was confirmed by the photoluminescence (PL) measurement.

Thermal and Electrical Conductivities of Porous Carbon-Coated Ceramic Fiber Composites

Porous ceramic fiber composites were coated with pyrolytic carbon by the decomposition of infiltrated phenolic resin in a nitrogen atmosphere at 800. The amount of carbon coating was varied to tailor the electrical conductivity of the carbon-coated composites. The electrical and thermal conductivity of the composites were measured at room temperature using a two-point method and a hot-wire one, respectively. Up to 30 wt% pyrolytic carbon, the electrical conductivity σ shows linearly increasing tendency and is fitted by the effective conductivity according to the parallel rule of a mixture σeff = ΣΧi ·σi with an effective conductivity of pyrolytic carbon σc= 0.42 S/cm. The thermal conductivity of the coated composites is in the range 0.05-0.08 W/mK and increases with carbon content.

Removal of Total Organic Carbon from Aqueous Phenol Solution Using Photocatalytic ZnO Nanopowders Prepared by Solution Combustion Method

ZnO nanopowders were prepared by Solution Combustion Method (SCM). The ZnO nanopowders synthesized using Zn(OH)2 and glycine as an oxidant and a fuel (with fuel/ oxidant ratio, F/O=0.8), showed excellent crystalline and photocatalytic characteristics. To confirm photocatalytic activities of the prepared ZnO powder, total organic carbon (TOC) was tried to remove from aqueous phenol solution. Surprisingly, SCM ZnO nanopowder shows 1.6 fold higher destruction rates of the organic pollutant than P-25 TiO2 nanopowder which is known as a kind of standard photocatalyst.

Preparation and Morphology of C/C-SiC Composites by Si Melt Infiltration

Two different types of carbon fibre bundles were used for filament winding to obtain C/C preforms. C/C-SiC composites were produced from the C/C preforms by a silicon melt infiltration technique. The effect of the type of carbon fibre bundle on the mechanical and thermal properties of the resultant C/C-SiC composites was compared. The spun fiber preform yields C/C-SiC composites of better mechanical properties than the unidirectional continuous fiber preform. The strength of the composites from the SFP was 1.8 times higher than that from the CFP. The flexural strength and the O-ring strength of the composites from the SFP with a density of 2.35 g/cm3 were about 160 MPa and 170 MPa, respectively.

Ultrasonic Spray Nozzle System with Piezoelectric Ceramics for Water Treatment Applications

A new type of ultrasonic spray nozzle was fabricated employing PZT and Pb-free piezoelectric device. The spray nozzle was designed to disperse chemicals in a water treatment mixing tank. The piezoelectrics were packaged in an aluminum case with silicone resin for the aqueous solution proof packaging. Chemicals were dispersed with high efficiency, and the ultrasonic fine particle spraying was able to reduce the chemicals consumption. The removal efficiencies of heterotrophic bacteria and Escherichia coli by chemicals injected using designed ultrasonic spray nozzle were higher than the conventional methods.