Papers by Author: Sung Gap Lee

Abstract: The V1.9W0.1O5 thin films deposited on Pt/Ti/SiO2/Si substrates by RF sputtering method exhibited fairly good TCR and dielectric properties. It was found that film crystallinity, dielectric properties, and TCR properties were strongly dependent upon the annealing temperature. The dielectric constants of the V1.9W0.1O5 thin films annealed at 300°C were 37.7, with a dielectric loss of 2.535, respectively. Also, the TCR values of the V1.9W0.1O5 thin films annealed at 300°C were about -3.7%/K.

Abstract: PZT(70/30) powder was prepared by a sol-gel method and PZT thick films were fabricated by the screen-printing method on the alumina substrates. The coating and drying procedure was repeated 4 times. And then the PZT(30/70) precusor solution was spin-coated on the PZT thick films. A concentration of a coating solution was 0.5 mol/L and the number of coating was varied from 0 to 6. The porosity decreased and the grain size increased with increasing the number of coatings. The thickness of the PZT-6(6: number of coatings) films was about 60~65μm. All PZT thick films showed the typical XRD patterns of a typical perovskite polycrystalline structure. The relative dielectric constant of the PZT-6 thick film was 540. The remanent polarization and coercive field of the PZT-6 film were 23.6 μC /cm2, 12.0 kV/cm, respectively.

Abstract: The BaTiO3/SrTiO3 heterolayered thick films were fabricated by two different methods – thick films of BaTiO3 by screen printing method on alumina substrates electrodes with Pt, thin films of SrTiO3 by the spin-coating method on BaTiO3 thick films and once more thick films of BaTiO3 by the screen printing method on SrTiO3 layer. The leakage current and the dielectric properties were investigated for effect of various stacking sequence of sol-gel-prepared SrTiO3 layer at interface of BaTiO3 thick films. The insertion of SrTiO3 interlayer yielded BaTiO3 thick films with homogeneous and dense grain structure with the number of SrTiO3 layers. The leakage current density of the BaTiO3/SrTiO3-7 film is less that 1.5
10-9 A/cm2 at 5 V.

Abstract: Ferroelectric Pb(Zr0.6Ti0.4)O3 (PZT(60/40)) powder was prepared by a sol-gel method and PZT thick films were fabricated by the screen-printing method on the alumina substrates. The Pt bottom electrodes were screen-printed on the alumina substrate. The PZT(60/40) thick films were annealed at 1050°C for 10min in PbO atmosphere. Pb(Zr0.4Ti0.6)O3 (PZT(40/60)) precursor solution, which prepared by sol-gel method, was spin-coated on the PZT thick films to obtain a densification. These PZT multilayered thick films were annealed at 650°C for 2 h in PbO atmosphere. The upper electrodes were fabricated by screen printing the Ag paste and then firing at 590°C for 10min. Its structural and electrical properties of the PZT thick films with the treatment of PZT(40/60) precursor solution coating were investigated. The coating and drying procedure was repeated 4 times. And then the PZT(40/60) precursor solution was spin-coated on the multilayered thick films. A concentration of a coating solution was 1.5 mol/L and the number of coating was repeated from 0~15. The porosity of the thick films was decreased with increasing the number of coatings. All PZT multilayered thick films showed the XRD patterns of typical peroveskite polycrystalline structure. The relative dielectric constant of the PZT-15 (15: number of solution coatings) thick film were 370. And the PZT-15 thick film shows the remanent polarization of 23.5 μC/cm2 and coercive field of 18.0 kV/cm, respectively.

Abstract: Ferroelectric PZT(70/30) thick films were fabricated by the hybrid technique adding the sol coating process to the screen-printing process to obtain a good densification. Structural and electrical properties of the thick films with the sol concentration were investigated. The relative dielectric constant increased and dielectric loss decreased with increasing the sol concentration, the values of the thick film coated with sol concentration of 1.5 M were 698 and 2.5 % at 1 kHz, respectively. And the remanent polarization and dielectric breakdown strength of the thick film coated with sol concentration of 1.5 M were about 38 μC/cm2 and 200 kV/cm, respectively. The leakage current densities were less than 10-8 A/cm2 at the applied electric field range of 0-100 kV/cm in all thick films.

Abstract: The currently commercialized cathode material for Li ion batteries such as LiCoO2 exhibited limit to further improve the performance of the batteries, since the employed screen printing method for cathode fabrication is difficult to reduce the thickness and control the microstructure of the oxide layer. In this studies, we have synthesized Li(Ni1-xCox)O2 thin film by utilizing Li-diffusion reaction on the surface of Ni-Co alloy substrates. For the preparation of Ni-Co alloy rod, Ni and 20at.%Co powder were mixed for 24hrs by ball milling, and then pressed into rod-shape by cold-isostatic pressing. The Ni-Co rods were sintered at 1100°C for 6hrs in the reducing atmosphere of Ar 96% and H2 4%. The sintered Ni-20at%Co rod was cold-rolled into tape at 5% reduction ratio with the final thickness of 100㎛, and the recrystallization heat treatment for the development of the cube texture of the rolled Ni alloy tape was carried out at 1000°C in Ar 96% and H2 4%. After thin layer of metallic Li was deposited on the surface of Ni-Co template using thermal evaporation method in the glove box, the Li/Ni-Co composite tape were heat-treated at 700~850°C for 1~2hrs in oxidizing atmosphere to induce Li-diffusion into Ni-Co substrate and Li(Ni0.8Co0.2)O2 phase formation. The phase evolution of Li(Ni0.8Co0.2)O2 was confirmed by X-ray diffraction and the grain size and morphology of the surface were analyzed by scanning electron microscopy and atomic force microscopy. Also the charge and discharge test were conducted to confirm the electrical characteristics of Li(Ni1-xCox)O2/Ni-Co thin film for the cathode application.

Abstract: LiNiO2 thin films for the application of cathode of the rechargeable battery were fabricated by Li ion diffusion on the surface oxidized NiO layer. Bi-axially textured Ni-tapes with 50 ~ 80 μm thickness were fabricated using cold rolling and annealing of Ni-rod prepared by cold isostatic pressing of Ni powder. Surface oxidation of Ni-tapes were conducted using tube furnace or line-focused infrared heater at 700 °C for 150 sec in flowing oxygen atmosphere, resulted in NiO layer with thickness of 400 and 800 μm, respectively. After Li was deposited on the NiO layer by thermal evaporation, LiNiO2 was formed by Li diffusion through the NiO layer during subsequent heat treatment using IR heater with various heat treatment conditions. IR-heating resulted in the smoother surface and finer grain size of NiO and LiNiO2 layer compared to the tube-furnace heating. The average grain size of LiNiO2 layer was 0.5~1 μm, which is much smaller than that of sol-gel processed LiNiO2. The reacted LiNiO2 region showed homogeneous composition throughout the thickness and did not show any noticeable defects frequently found in the solid state reacted LiNiO2, but crack and delamination between the reacted LiNiO2 and Ni occurred as the reaction time increased above 4hrs.

Abstract: Superconducting YBa2Cu3O7-δ
(YBCO) films were grown on MgO single crystalline substrates using a BaZrO3 (BZO) buffer layer deposited by a pulsed laser deposition (PLD). Deposition condition has been optimized to obtain good epitaxial BZO film followed by deposition of YBCO superconducting films. The crystallinity and microstructure of epitaxial YBCO/ BZO/ MgO (00l) films were investigated by a two-dimensional x-ray diffraction and a field emission scanning electron microscope. The in-plane (φ-scan) measurements for the BZO films (200 ~ 500 nm thick) grown on MgO substrates revealed a narrow full width half maximum (0φ = 2o). The XRD results exhibited that YBCO films with a BZO buffer layer were well oriented in the [00l] direction perpendicular to the substrate surface. The BZO films also showed homogeneous and dense surface morphologies. By the deposition of a subsequent BZO buffer layer, YBCO was grown epitaxially on MgO with results showing a critical current density (Jc) of ~ 3.3 × 106 A/cm2 and a critical temperature (Tc) of 86 K.

Abstract: To prevent the shrinkages by the densification during the application of unfired Al2O3-C refractories or Al2O3 castables in steel making conditions, MgO was added as aggregate or matrix powder and the expansion caused by spinel formation was studied. Because the spinel was formed at the contacting areas between Al2O3 and MgO particles and the volume of in-situ formed spinel increased more abnormally at the side of Al2O3 particles than MgO, the addition of MgO aggregates was not recommendable due to the formation of large voids around the MgO aggregates. Nevertheless, corrosion resistance was increased with the amount of fine MgO added, and the finer MgO powder added, the better residual expansion and minute structure formation was observed. In the contrary, in case of castables volume expansion due to spinel formation was not obvious because the degree of densification was less than high-pressure formed refractories. But CA6 phase would not form around alumina aggregates during corrosion so the corrosion resistance was much more enhanced.

Abstract: Ferroelectric PZT heterolayered thick films were fabricated by the alkoxide-based sol-gel method. PZT(20/80) and PZT(80/20) paste were made and alternately screen-printed on the alumina substrates. The coating and drying procedure was repeated 4 times to form the heterolayered thick films. The thickness of the PZT heterolayered thick films was approximately 60 μm. All PZT thick films showed the typical XRD patterns of a perovskite polycrystalline structure. The relative dielectric constant and the dielectric loss of the PZT thick films sintered at 1050oC were 283 and 1.90%, respectively.