Low Thermal Conductivity and Radiation of Porous Ceramics

This study used colloidal dispersion and direct foaming method to distributed bubbles uniformly in ceramic slurry. After microwave drying and appropriate sintering, the porous ceramics with porosity 85 ~ 95 % were made. The effects of material and porous structure on thermal conductivity are investigated in this study. Porous Al2O3, ZrO2 and mica with porosity higher than 85 % are foamed by the direct foaming method with SDS and valeric acid as surfactants. The gas cells of porous ceramics in an average bubble size with the range of 17 um to 80 um were characterized by scanning electron microscope equipped with energy dispersive spectroscopy (SEM-EDX). Two different cell walls with dense and non-dense profiles exist in the porous samples. Thermal mechanical analysis (TMA) was used to analyze the sintering behaviors of porous ceramics with different porosities. Light scattering between 1000 cm-1 and 4000 cm-1 were investigated by Fourier Transform Infrared Spectroscopy (FTIR). The instrumental set-up for measuring thermal conductivity by a hot-wire method was calibrated by the standard sample. The thermal conductivities of porous ceramics with different porosity, grain size, cell walls, and defects are observed and compared to each other. Consequently, small k value of 0.05 W/m*oC at room temperature and 0.17 W/m*oC at 800oC of one Al2O3 foam with the porosity of 95 % can be reached in this study.