Conductivity relaxation studies on La0.5Sr0.5Co0.8Ga0.2O3-δ

The surface exchange coefficient, ki, of the mixed ionic electronic conductor perovskite, La0.5Sr0.5Co0.8Ga0.2O3-delta, was determined in the temperature range 600-900 degrees C by electrical conductivity relaxation on thin plate and long bar samples, using four probe de conductivity measurements. The normalized conductivity could be fitted by the equation corresponding to surface limited relaxation, (sigma(t) - sigma(1))/(sigma(2) - sigma(1)) = 1 - exp(-t / tau), where tau = l(1)l(2) / [(l(1) + l(2))Gamma k(i)], sigma(t), sigma(1) and sigma(2) are the time dependent, initial and final conductivities, Gamma is the thermodynamic factor and 2l(1) and 2l(2) are the lateral sides of the bar. Numerical simulation shows that the shape of the relaxation curve for the plate geometry changes only little in going from surface limited to diffusion limited transport, the change is significantly larger for the bar geometry.