Papers by Author: W.K. An

Abstract: The Al-40Si alloy was modified by 0.5 wt% Sr addition and heat treated using the solution treatment and aging treatment. Its deforming capacity and hardness variance were investigated. The microstructures of the deformed samples were analyzed using optical microscope. The results show that the hardness increases with increasing of the deformation degree. When the deformation ratio exceeds 10.1 %, the hardness sharply increases. When the deformation ratio exceeds 20.4 %, the hardness maintains almost unchanged. In addition, although the cracks are not observed on the surface when the deformation ratio is less than 14.6 %, the primary Si phase is cracked. Thus the critical deformation ratio of the studied Al-40Si alloy is 10.1 %.

Abstract: The electrical properties and Microstructures of Tb-doped bismuth titanate (Bi3.3Tb0.6Ti3O12) ceramic were investigated. XRD analyses revealed that the sample had Bi-layered perovskite structure. SEM micrographs showed randomly oriented and plate-like morphology. The remanent polarization (Pr) and coercive field (Ec) of Bi3.3Tb0.6Ti3O12 ceramic are above 25 μC/cm2 and 80 KV/cm, respectively.

Abstract: Sm-doped bismuth titanate and random oriented Bi4-xCexTi3O12 (BCT) thin films were fabricated on Pt/Ti/SiO2/Si substrates rf magnetron sputtering technique. The structures and the ferroelectric properties of the films were investigated. Ce doping leads to a marked improvement in the remanent polarization (Pr) and the coercive field (Ec). At the applied electric field of 100 kV/cm, Pr and Ec of the BCT (x = 0.8) film annealed at 650 oC are 20.5 μC/cm2 and 60 KV/cm, respectively. However, after 3 × 1010 switching cycles, 20% degradation of 2Pr is observed in the film.

Abstract: The electrical properties of Bi3.25Dy0.75Ti3O12 (BDT) and Bi3.25Gd0.75Ti3O12 (BGT) ceramics were investigated. The current-voltage curve of the BGT sample exhibits a negative differential resistance behavior, whereas that of the BDT sample exhibits a simple ohmic behavior. The impedance spectrum of the BDT and BGT samples indicate that both consist of semiconducting grain and moderately insulating grain boundary regions. XRD, SEM and EPMA analyses reveal crystalline phase characterized by a Bi-layered perovskite structure of Bi4Ti3O12 and the distribution of every element is uniform. Both BDT and BGT samples exhibit randomly oriented and plate-like morphology.

Abstract: Pr-doped bismuth titanate (BixPryTi3O12: BPT) thin films with random orientation were fabricated on Pt/Ti/SiO2/Si substrates by rf magnetron sputtering technique, and the structures and Raman spectra of the films were investigated. XRD studies indicated that all of the BPT films consist of single phase of a bismuth-layered structure, showing a highly (117) oriented preferential growth with a minor fraction of (00l) orientation. For an increasing degree of Pr doping, Raman spectra studies revealed a substantial hardening of the vibration involving Bi atoms at the perovskite A site, whereas the Bi mode at the Bi2O2 layer is negligibly changed. From a comparison with a simple mass consideration, we identify a precise cation distribution, indicating a pronounced site selectivity of Pr ions for the A site for y ~ 1.2.

Abstract: Tb-doped bismuth titanate (BixTbyTi3O12: BTT) and pure Bi4Ti3O12 (BIT) thin films with random orientation were fabricated on Pt/Ti/SiO2/Si substrates by rf magnetron sputtering technique. These samples had polycrystalline Bi-layered perovskite structure without preferred orientation, and consisted of well-developed rod-like grains with random orientation. Tb doping into BIT caused a large shift of the Curie temperature (TC) from 675°C to lower temperature. The experimental results indicated that Tb doping into BIT result in a remarkable improvement in dielectric property.