Publication Date:
2014-05-16
Description:
The electrical conductivity of 2/1-mullite (approximate composition 2Al 2 O 3 ·SiO 2 ) was measured using plane parallel, polished plates cut perpendicular to [100], [010], and [001] from a large single crystal grown by the Czochralski method. Impedance spectra were recorded in the 1 Hz to 1 MHz frequency range at temperatures from 550 to 1400 °C in air. The conductivity vs. temperature curves display changes of their slope between 850 and 950 °C depending on the crystallographical direction. The low-temperature region ( T 〈 850 °C) of conductivity is characterized by low-electrical conductivities ( av 5.4 x 10 –9 –1 cm –1 , average conductivity at 550 °C) with [010] 〉 [100] 〉 [001] and low-activation energies (0.66 eV, average value). In the high-temperature region ( T 〉 950 °C) the electrical conductivity is significantly higher ( av 1.1 x 10 –5 –1 cm –1 , average conductivity at 1400 °C) with [001] 〉 [100] [010] , and with higher activation energies (1.6 eV). While the conductivity in the low-temperature region essentially is electronic, ion conductivity dominates the conductivity in the high-temperature region. We believe that the ionic conductivity is essentially due to hopping of O atoms from structural sites linking the tetrahedral double chains in mullite toward adjacent oxygen vacancies especially in c -axis direction. These oxygen hoppings are associated with complex structural re-arrangements, which control and slow down the velocity of the processes. Thus the electrical conductivity of mullite at high temperature is much lower than, e.g., that of Y-doped zirconia, but is significantly higher than that of α-alumina.
Print ISSN:
0003-004X
Electronic ISSN:
1945-3027
Topics:
Geosciences
Permalink