2.3. Impedance spectroscopy
Electrical conductivity measurements of pellets (~0.15 cm in thickness
and ~1 cm in diameter) of the sol–gel prepared powder (sintered
with and without pressure) were performed in air using Pt-electrodes
(Pt-paste obtained from SPI supplies-structure probe Inc., SPI-Chem™
Conductive Platinum Paint/Suspension, USA, cured at 1000 °C in air
for 1 h to remove the organic binders) and employing AC impedance
and gain phase analyzer (SI model no. 1260) (0.1 Hz–15 MHz). A
two probe electrochemical cell was used. In Fig. 4, the bulk and grain
boundary contributions obtained at 400 °C for LSO, LASO, and LFSO
compositions are shown. It can be observed that the grain boundary
conductivity is lower than the bulk conductivity, and grain boundaries
act as barriers for diffusion of oxide ions.The influence of the grain
size on the conductivity of the used materials has been published elsewhere
[59].
The applied ac voltage was 10 mV. Conductivity measurements in a
frequency range of 1–10MHzwere performed in air over a temperature
range 190–820 °C.