Electrical transport properties in zirconia/alumina functionally graded materials

Functionally graded materials (FGMs) are promising candidates for the fabrication of technological components, not only as structural devices, but also in electrochemical ones, such as solid oxide fuel cells (SOFC), or high-efficiency hybrid direct energy conversion systems. In the present work FGMs were prepared by the sequential slip casting technique, starting with an yttria tetragonal zirconia polycrystalline layer and increasing subsequently the amount of Al2O3 in the following layers. Electrochemical impedance spectroscopy (EIS) analysis was used to evaluate the electrical characteristics of these materials and to compare with those of the monolithic compacts. In general, it was observed that the FGM conductivity is ruled by the conductivity of the layer which contains the highest amount of alumina blocking particles. By EIS no electrical interfaces between adjoining layers were detected and, accordingly, no specific electric ohmic losses were observed. The conductivity of the FGMs is close to that calculated using the normalized thicknesses and the alumina volume fractions of the layers after measuring the conductivity of the monolithic materials with the same composition to what correspond to that of the final layer in the FGM. These results suggest that the gradient structure can be used to control the oxygen vacancy motion, and then applied in electrochemical devices.