IONIC CONDUCTIVITY IN MULLITE AND MULLITE TYPE COMPOUNDS

摘要

Mullite, Al_(4.8)Si_(1.2)O_(9.6), and mullite type componds Bi_2M_4O_9 with M = Fe, Al were investigated by impedance spectroscopy between 1 Hz and 1 MHz and temperatures between 200°C and 800°C. The data reveal universal dielectric response (UDR) type behaviour which could be separated as σ(v) = σ_0 + Av~s and ε(v) = ε_(st) + Av~(s-1)tan(sπ/2)/ε_0, however, with the s parameter dependent on temperature and frequency. Specific σ_0 values of the crystal plates could be extracted which are about 10~(-8) S/cm for mullite, 10~(-7) S/cm for Bi_2Al_4O_9 and 10~(-6) S/cm for Bi_2Fe_4O_9 taken at 500°C. The main peak in the Modulus representation M" is given by 2πv_c = σ_0/ ε_(st) ε_0 identifying the net rate of thermally activated excitations 2πv_c = 2πv_0exp(-E_a/kT) of relevant charge carriers. Ea of about 1.3 eV and 1 eV are obtained for Bi_2M_4O_9 and mullite, respectively. With increasing temperature effects related to ion blocking at electrodes are observed towards lower frequencies. It is discussed that for mullite σ_0 is related to impurity charge carriers (Na~+, H~+, etc.) whereas for Bi_2Al_4O_9 and Bi_2Fe_4O_9 "intrinsic" diffusion of Al, Fe via Frenkel type defects may not be ruled out.