Grain boundary conductivity activation energies

Grain boundaries offer some impedance to sodium ion conduction (i.e., approximately a factor of five increase for polycrystalline jS -alumina at 300 C) " . They also increase the activation energy for conduction by approximate factors of 1.6-2 depending on the temperature and grain size. Sodium ion grain boundary conduction is dominant in polycrystalline 8"-alumina when the grain size is very small (< 1-2 pm) and when the temperature is below 100°C. 

A and B. Conductivity relaxations, related to the microstructure of the composite. Their activation energies are about 1 eV, similar to those of the grain or grain boundary conductivities and are independent of the oxygen partial pressure (Po ). One or both of these arcs may be absent in some composite cathodes, but are absent with point contact electrodes. 

The fabrication procedure affects the product s microstructure including grain size, grain-boundary width, and porosity. In addition, different procedures introduce various amounts of impurities to the product. Therefore, the electrical conductivity and activation energy are affected by the fabrication procedure since, as mentioned above,... 

The fact that in the case of the bicrystal in Figure 54 the effective thickness calculated from the low-frequency semicircle is very much greater than expected from the width of the boundary, while the activation energy is almost equal to that of the bulk, points towards a frequently overlooked complication, namely to current-constriction effects. Such constriction effects occur287 when the crystal grains are not ideally sintered together, if pores or second phases are included, and interrupt the lateral conductivity of boundaries, as is the case for inhomogeneous electrode contacts. 

Surfaces of crystals and the grain boundaries may have disordered regions where proton transport is facilitated. Adsorbed water or electrolyte solution trapped between the grains will also increase the conductivity and give rise to false conclusions concerning the solid-state nature of conductivity. Surface or liquidlike conductivity has a low activation energy ( a 0.1-0.4eV), approaching the of aqueous acidic... 

---