The Science of Engineering Ceramics III

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Authors: Young Wook Kim, Shin Han Kim, Chul B. Park, Hai Doo Kim
Abstract: Recently, a novel processing route for fabricating microcellular ceramics has been developed. The strategy for making the microcellular ceramics involves: (i) forming some shapes containing a mixture of preceramic polymer, expandable microspheres and optional fillers by a conventional ceramic forming method, (ii) foaming the compact by heating, (iii) cross-linking the foamed body, and (iv) transforming the foamed body into microcellular ceramics by pyrolysis. The flexural strength and compressive strengths of the microcellular ceramics were investigated; values up to 30 MPa and 100 MPa, respectively, were obtained at room temperature. The superior mechanical properties were attributed to homogeneous distribution of cells in microcellular ceramics.
Authors: Jun Young Park, Jong Ho Lee, Joo Sun Kim, Hae Won Lee
Abstract: A novel Ni-YSZ anode with interpenetrating phase composite (IPC) structure was developed using NiO-YSZ core-shell composite powder and evaluated in terms of microstructure, electrical conductivity, thermal expansion and flexural strength. In comparison to conventional anode, the anodic performance of IPC anode appeared to be more desirable for improving structural reliability of SOFC unit cells and stacks. This study reveals that the anodic performance of IPC anode can be readily tailored by controlling core-shell composite powder particles.
Authors: Yoshinobu Fujishiro, Kouich Hamamoto, Ramalinga V. Mangalaraja, Masanobu Awano
Abstract: Micro-tube type Gd-doped CeO2(CGO) reactor were prepared by hydrolysis of metal acetate gel at room temperature. The size of anode tube was ca. 0.8mm in diameter, and length was 20mm after sintering at 1400°C. DC conductivity of the prepared dense CGO cell consisting of 500nm grains was about 10 Ohm-cm at 600°C. Electrical power density of the micro-tube fuel cell, which was coated both CGO electrolyte and CGO/La0.6Sr0.4Co0.2Fe0.8O3 cathode on the surface of 40vol%NiO-CGO anode tube, was about 900mW/cm2 at 600°C under 30ml/min H2 gas flowing.
Authors: Tae Ho Shin, Ji Heang Yu, Shi Woo Lee, In Sub Han, Sang Kuk Woo, Byung Koog Jang, Sang Hoon Hyun
Abstract: Yttria stabilized zirconia (YSZ) films with the thickness of up to 12 μm were prepared on alumina and NiO-YSZ substrates by electron beam physical vapor deposition (EB-PVD). The films showed nano-scaled columnar structures depending on the substrate temperature. Electrical conductivity of the YSZ films on alumina was also investigated at the temperature between 700 and 1000oC in oxidizing atmosphere. High activation energy of the conductivity (>1.03eV) indicated that the conduction via grain boundary controlled the ionic conduction in the films prepared by EB-PVD. La0.6Sr0.4CoO3-δ as a cathode was applied on the YSZ/NiO-YSZ in order to evaluate the performance of the YSZ electrolyte.
Authors: Jae Kwang Kim, Kyung Hun Kim, Yong Ho Choa, Jong Won Yoon, Kwang Bo Shim
Abstract: Dense 8mol% yttria-stabilized zirconia (8YSZ) consisting of submicrometer-sized grains was prepared using spark plasma sintering (SPS) along with Al2O3 additives. The starting powder with average particle size of 50nm was densified to 98% of the relative density with short sintering time (5min) at 1200 while preserving a submicrometer grain size. The fracture toughness and bending strength showed maximum values of 2.54MPam1/2 and 380MPa at 2vol% alumina-added 8YSZ, due mainly to the higher relative density and small grain size. The electrical conductivity of 2vol% alumina-added 8YSZ was 0.0278 S/cm at 700 in airThus, alumina additions in 8YSZ using the SPS method are an effective process to improve the mechanical strength and electrical conductivity.
Authors: Jae Chun Lee, Sung Park, Jun Suh Yu, Jong Ho Lee, Joo Sun Kim, Hae Won Lee
Abstract: Porous composites containing ceramic fiber have been developed for the fabrication of SOFC seals. They were fabricated using glass powder and alumino-silicate chopped fibers. Effect of mixing ratios of ceramic fiber and glass on the leak rates and strength of the composite seals was investigated. In addition, seal performance of commercial glasses was compared with that of SiO2-BaO-B2O3 glass synthesized in this work. The leak rate of the composite seals containing 55 vol% glass was seven times higher than the one containing 75 vol% glass. The flexural strength of the composite seals was reduced to one fourth of the initial value as the porosity increased from 1 to 29%. The incorporation of alumino-silicate chopped fibers into a sealing glass degraded room temperature strength and increased leak rates due to increase in porosity with increasing fiber content. The viscosity of glass at the seal test temperature is presumed to affect the leak rate of the glass seal.

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