Electrical Conductivity of Mullite Ceramics

The electrical conductivity of a lab-produced homogeneous mullite ceramic sintered at 1625 degrees C for 10h with low porosity was measured by impedance spectroscopy in the 0.01Hz to 1MHz frequency range at temperatures between 300 degrees C and 1400 degrees C in air. The electrical conductivity of the mullite ceramic is low at 300 degrees C (approximate to 0.5x10-9Scm-1), typical for a ceramic insulator. Up to approximate to 800 degrees C, the conductivity only slightly increases (approximate to 0.5x10-6Scm-1 at 800 degrees C) corresponding to a relatively low activation energy (0.68eV) of the process. Above approximate to 800 degrees C, the temperature-dependent increase in the electrical conductivity is higher (approximate to 10-5Scm-1 at 1400 degrees C), which goes along with a higher activation energy (1.14eV). The electrical conductivity of the mullite ceramic and its temperature-dependence are compared with prior studies. The conductivity of polycrystalline mullite is found to lie in-between those of the strong insulator -alumina and the excellent ion conductor Y-doped zirconia. The electrical conductivity of the mullite ceramic in the low-temperature field (< approximate to 800 degrees C) is approximately one order of magnitude higher than that of the mullite single crystals. This difference is essentially attributed to electronic grain-boundary conductivity in the polycrystalline ceramic material. The electronic grain-boundary conductivity may be triggered by defects at grain boundaries. At high temperatures, above approximate to 800 degrees C, and up to 1400 degrees C gradually increasing ionic oxygen conductivity dominates.