Papers by Author: Geun Hee Lee

Abstract: Implantable neural prostheses form hybrid interfaces with biological constructs, and the application of electrical fields can restore functions of patients with neurological damages. The various stoichiometric compositions of iridium oxide were synthesized using reactive magnetron sputtering. The charge injection behavior of iridium oxide deposited with an O2/Ar ratio of 0.5 was similar to pure Ir. The charge density of iridium oxide increased with increasing O2/Ar ratio, and increasing thickness of iridium oxide.

Abstract: Al2O3 compacts with various phases were prepared by hydrolysis and spark plasma sintering (SPS) process of Al powder. The bayerite (
-Al(OH)3) phase was derived by hydrolysis of commercial Al powder with micron size, whereas the bohemite (AlO(OH)) phase was obtained by hydrolysis of nano Al powder synthesized by pulsed wire evaporation (PWE) method. Compaction as well as dehydration of both bayerite and bohemite was carried out simultaneously by SPS method, which is used to fabricate nano powder into dense compacts with a rapid heating rate of about 100 °C per min. under the pressure of 50 MPa. After compaction in the temperature ranges from 350 °C to 1100 °C, the bayerite and bohemite phases change into various alumina phases depending on the compaction temperatures. The bayerite shows the phase transition of Al(OH)3  -Al2O3  -Al2O3  -Al2O3 sequences. On the other hand, the bohemite experiences the phase transition from AlO(OH) to
-Al2O3 at 350 °C showing AlO(OH) 
-Al2O3  -Al2O3  -Al2O3  -Al2O3 sequences.

Abstract: Magnetic Pulse Compaction (MPC), as a dynamic compaction, can be possible to reach higher relative density of nano metallic compacts owing to sufficiently high pressure and adiabatic heating in very short duration of an order of µsec. The present work is concerned with the magnetic pulsed compaction of the nano-sized aluminum powders, which particle size was a range of 50 ~ 100 nm passivated in air. The compaction pressure was 1.5 GPa for 300 µsec in the temperature range from 20°C to 500°C. The grain size of compacts was maintained less than 50 nm, which was analyzed by X-ray diffraction (XRD) using Scherrer method. From the calculation of adiabatic heat and of pressure induced by thermal expansion, and the observation by transmission electron microscopy (TEM), it was found that Al₂O₃ could be broken and dispersed with a few nano-meter sizes in the Al matrix and that the ultra fine and uniform bulk structure was maintained up to 400°C of compaction temperature.e