Papers by Author: Sang Im Yoo

Abstract: We report important factors affecting the dielectric properties of CaCu3Ti4O12 (CCTO) polycrystalline ceramics prepared by the conventional solid-state ceramic process. The relative dielectric constants (εr) up to several thousands (~ 3,000 at 1 kHz) were gradually increased with increasing the sintered density of samples in the case that no exaggerated grain growth occurred. An abrupt increase in εr values were, however, accompanied by the formation of abnormally grown large grains, and thus with increasing the population of abnormally grown grains, which could be achieved by a prolonged sintering at 1060°C, the εr values were remarkably increased from several thousands to ~105. Optimally processed CCTO sample exhibited a very high εr of ~ 90,000 at 1 kHz.

Abstract: We fabricated bi-axially textured pure Ni and Ni–5at.%W (Ni-5W) alloy tapes for the application of rechargeable battery cathode and coated superconductor, where the controlled microstructure of the substrate plays a critical role. The sintered Ni or Ni-W rods were cold-rolled into the thin tapes of 80 ~ 100 µm thickness, and the tapes were heat-treated for texture development with a line focused infrared heater. The temperature was maintained at 800 ~ 1050 °C, using a 1kW double ended linear halogen lamp in 96%Ar-4%H2 atmosphere. The (2 0 0) texture of Ni tape was successfully formed through optimization of the recrystallization infrared heating condition for the cold rolled Ni tapes. The full width half maximum of the Ni tapes was less than 10o, and the grain size was 20 ~ 40 µm. Focused IR-heating resulted in a better texture development and smaller grain size compared to the conventional resistive heating because of the steep temperature gradient.