Development of Multi-Functional NITE-Porous SiC for Ceramic Insulators

Full text of publication follows: Recently, porous silicon carbide (SiC) ceramics are considered as functional materials for stainable and advanced energy systems because of their low thermal conductivity, low electrical conductivity, low thermal-expansion coefficient, and good thermal-shock resistance as well as excellent mechanical and chemical stability at elevated temperature. A number of manufacturing approaches have been applied to fabricate porous SiC ceramics including polymer pyrolysis, oxidation bonding, and reaction bonding technique. However, their processes are complicated and conventional porous SiC shows insufficient mechanical and chemical stability under high temperature environment. Therefore, from the view point of safety and stability, it is necessary to develop a simple fabrication method. In this study, porous SiC ceramics have been fabricated based on the Nano Infiltration Transient Eutectic (NITE) process, which is developed as a processing technique for the high performance a SiC fiber reinforced SiC matrix composite. The porosity was calculated from relative density and theoretical density, which were obtained by the rule of mixture. The pore shape and size distribution were examined by optical microscopy and scanning electron microscopy. Mechanical properties were investigated using three-point bend test and tensile test. Thermal conductivity was measured by the laser flash method at room temperature to 900 deg. C. Porosity of the NITE-porous SiC ceramics could be controlled at less than ± 0.5 %. Mean pore size was 0.5 μm regardless of porosity. Flexural strength of the NITE-porous SiC ceramics, where spherical pores were introduced to the porosity of 50 % was about 170 MPa. The NUE-porous SiC ceramics exhibited a substantially high strength in comparison with other conventional porous SiC ceramics, due to its robust microstructure consisted of spherical pores. Thermal conductivity of the NITE-porous SiC ceramics which had porosity of 50 % was about 6 W/m/K. Furthermore, thermal conductivity, k, decreased with increasing the test temperature: k = 6 W/m/K at room temperature, dropping to k = 4 W/m/K at 900 deg. C. The porosity, pore shape, size and their distribution effects on mechanical and thermal insulation properties will be discussed. (authors)