Microstructure blocking effects versus effective medium theories in YSZ

Analogies and differences between the experimental electrical responses of poles and Al2O3 inclusions in YSZ and the corresponding predictions by the basic Effective Medium Theories are examined If is shown that under restricted conditions, the EMT predicts a 'two semicircle' impedance diagram for a composite. Systematic variations of the EMT model parameters demonstrate that, over fairly wide variation intervals, each semicircle can be viewed as a characteristic response of the col responding component altered by the presence of the other. A comparison with the experimental properties reveals marked differences. Specific experimental features such as the disappearance of the blocking process when the matrix conductivity becomes sufficiently high or the influence of the inclusion grain size on the magnitude of the corresponding semicircle cannot be explained by the EMT. Numerical calculations applied to real cases clearly show that the blocking process semicircle is an additional feature with respect to the EMT predictions. It has been interpreted as experimental evidence for a distortion of the current lines by the microstructure defects. It is a specific property of low-conductivity materials. (C) 1997 Elsevier Science Limited.