Papers by Author: Byung Dong Hahn

Abstract: Lead zinc niobate (PZN) added lead zirconate titanate (PZT) thick films with thickness of 5~10 μm were fabricated on silicon and sapphire substrates using aerosol deposition method. The contents of PZN were varied from 0, 20 and 40 %. The as deposited film had fairly dense microstructure without any crack, and showed only a perovskite single phase formed with nano-sized grains. The as-deposited films on silicon were annealed at temperatures of 700oC, and the films deposited on sapphire were annealed at 900oC in the electrical furnace. The effects of PZN addition on the microstructural evolution were observed using FE-SEM and HR-TEM, and dielectric and ferroelectric properties of the films were characterized using impedance analyzer and Sawyer-Tower circuit, respectively. The PZN added PZT film showed poor electrical properties than pure PZT film when the films were coated on silicon substrate and annealed at 700oC, on the other hand, the PZN added PZT film showed higher remanent polarization and dielectric constant values then pure PZT film when the films were coated on sapphire and annealed at 900oC. The ferroelectric and dielectric characteristics of 20% PZN added PZT films annealed at 900oC were comparable with the values obtained from bulk ceramic specimen with same composition sintered at 1200oC.

Abstract: Sintered reaction bonded silicon nitride with aligned whisker seeds was prepared by tape casting silicon slurry with 5 wt% b-Si3N4 whisker seeds followed by nitridation and sintering. Three different sintering additives were used for the samples; 7 wt% Y2O3, 6 wt% Y2O3 + 1 wt% Al2O3 and 5 wt% Y2O3 + 2 wt% Al2O3. The sample with 5 wt% Y2O3 + 2 wt% Al2O3 showed the fastest a to b phase transformation after nitridation and the highest fracture toughness and flexural strength after gas pressure sintering among the samples. It also had finer microstructure than the other samples after sintering at 2248 K and at 2273 K. The finer microstructure was related to the faster phase transformation after nitridation, which resulted in the higher flexural strength.

Abstract: Silicon nitride samples without and with 3 wt% of the aligned b-silicon nitride whisker seeds were prepared with 8.2 wt% Er2O3 and 1.9 wt% AlN. After sintering at 2148 K for 4h, the samples exhibited densities higher than 99.5% TD. The microstructures and properties of the samples were compared with those of the samples sintered with 4.8 wt% Y2O3 and 2.2 wt% Al2O3 at 2273 K for 4h. For samples without the whiskers, the sample with 4.8 wt% Y2O3 + 2.2 wt% Al2O3 had coarser microstructures than those with with 8.2 wt% Er2O3 + 1.9 wt% AlN. However, the samples with the whisker seeds, the former sample appeared to have only slightly larger grains than the latter sample in spite of the significant difference in the sintering temperatures. For the samples without the whisker seeds, the room temperature flexural strength was higher for the sample with Er2O3 + AlN. However, for the samples with the aligned whisker seeds, the sample with Y2O3 + Al2O3 exhibited higher room temperature flexural strength than that with Er2O3 + AlN although the average grain width of the former sample was larger than that of the latter sample. In case of the high temperature flexural strength at 1673 K, the flexural strengths of the samples with the whisker seeds were higher than double the strengths of the samples without the whisker seeds. For samples without the whisker seeds, the sample with Er2O3 + AlN exhibited better mechanical properties than that with Y2O3 + Al2O3. However, for the samples with the aligned whisker seeds, the sample with Y2O3 + Al2O3 exhibited better mechanical properties than those with Er2O3 + AlN. The results were explained in terms of the microstructures of the samples.