Preparation of Porous Yttria-Stabilized Zirconia Ceramics By Thermal Removal of Potassium Iodide

Near to full skeletal density ZrO₂:8 mol% Y₂O₃ polycrystalline ceramics with different pore content were prepared by mixing different amounts of KI and thermally removing them during sintering at 1400^oC. That removal of the alkali halide was evaluated by thermogravimetric and dilatometric measurements from room temperature to 1500^oC.

                The sintered pellets were analyzed by X-ray diffraction and X-ray fluorescence (XRF) for structural phase identification and KI content, respectively. Images of the polished and thermally etched surfaces were obtained by scanning probe microscopy (SPM) and scanning electron microscopy (SEM) for evaluation of average pore content and distribution. Electrochemical impedance spectroscopy measurements were carried out in the 300-450^oC range in the 5 Hz-10 MHz frequency range.

                 The thermal removal of KI starts at its melting point, ~ 680^oC and ends at ~ 900^oC. No traces of KI are found by XRF analysis in sintered pellets.

                 The final linear shrinkage value is higher for compositions without KI and the temperature of maximum shrinkage rate increases for increasing KI content.

                 The total oxide ion electrical resistivity, extracted from the impedance plots, also increases for increasing KI content.

                 The SPM images observed at polished and thermally etched surfaces of sintered pellets show that the larger is the amount of KI addition, the larger is the average pore size and the smaller is the average grain size.

                 The effect of KI addition before sintering leads, upon sintering, dense porous ceramics with high skeletal density. Moreover, grain growth is inhibited.

                 The impedance spectroscopy results allowed for the evaluation of the bulk and the grain boundary conductivities. The increase in both contributions with the increase in KI addition was correlated to the observation by scanning probe microscopy of the increase of intragranular as well as of intergranular porosity. Other parameters determined after the deconvolution of the impedance plots, namely, the blocking factor, the frequency factor and the descentralization angle showed that the impedance spectroscopy technique is an important tool for the characterization of porous ionic conducting solids.