Papers by Author: Dae Jin Yang

Abstract: Titanium of 99.7% purity was anodized in 1M potassium phosphate monobasic (KH2PO4) water solution with 0.15M NH4F. Titanium oxide nanotubes were fabricated at anodization potential of 20 V and 4.64 pH. To control the pH of the solution, we have added weak acid such as citric acid because it has three dissociation constants (pKa) of 3.09, 4.75, and 5.41. Citric acid was very useful to control the pH of the 1M KH2PO4 water electrolyte solution within 3 to 5. The diameter and length of the titanium oxide nanotubes were independent on anodization time. The diameter of 120 nm and length of 2.8 μm at anodization time of 5 hrs were observed by field emission scanning electron microscope (FESEM). Undesired thin oxide layer blocking the top of titanium oxide nanotubes was wiped out by increasing the anodization potential with the multi step voltage by 1 V reached to 25 V. The titanium oxide nanotubes having a very large surface area are very attractive for the battery, gas sensor, photocatalytic application, and biomaterials.

Abstract: Lead zirconate titanate (Pb(Zr0.48Ti0.52)O3 or PZT) films were grown on platinized silicon wafers (Pt/SiO2/Si) by d.c. reactive sputtering method with multi targets. The Pb content of PZT films has been widely recognized as affecting not only the phase formation and microstructure but also the dielectric and ferroelectric properties. Pb content of PZT films was controlled by the variation of Pb target current. The relation between Pb content and Pb target current was expressed as y=0.89x-11.09. The x and y are Pb target current and Pb content, respectively. The pyrochlore phase was transformed to perovskite phase as Pb content was increased. This phase transformation improved the ferroelectric properties of PZT films. In PZT films with perovskite phase, fatigue properties were not improved with excess Pb content. Fatigue properties of PZT films began to be fatigued after 106 switching cycles and coincided with the typical PZT fatigue behavior. Excess Pb content (Pb vacancy) did not affect the fatigue properties of PZT films.

Abstract: Thin-film titania templates were fabricated under different processing conditions. The dependencies of pore morphology and pore formation rate on process parameters were evaluated. It was found that under optimized electrolyte condition and anodizing voltage, a self-organized nanostructure consisting of porous TiO2 was obtained. 1 M Na2SO4 solution with the addition of 0.35 wt.% NaF and voltage potential of 30V were used. SEM images of templates showed very thin layer of ~70 nm thickness with worm-like pores. Pore diameter and average pore spacing of center to center on the surface were ~10 nm and 20 nm, respectively. The nanoporous TiO2 thin film will be very useful gas sensing and photocatalytic materials due to their large surface areas and high reactivities.

Abstract: In this paper, composites with Tb:Dy ratio of 0.30:0.70 and 0.33:0.67 were fabricated by polymer infiltration method. Both maximum d33 and strain of composites with different Tb/Dy ratio (0.33:0.67, 0.30:0.70) were investigated as a function of the volume fraction of RFe2 phase. The maximum d33 and the strain of composites with higher Tb:Dy ratio (higher magnetocrystalline anisotropy) showed higher than that of composites with lower one as volume fraction of RFe2 increased.