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The spatially-resolved local impedance can be measured by ‘Millicontact Impedance Spectroscopy (MIS)’, which involves the use of an array of sub-millimeter-scale electrodes. In stabilized zirconia and gadolinia-doped ceria electrolytes with highly resistive grain-boundaries, MIS could measure the local impedance precisely and revealed the spatially-resolved changes in the grain-interior and grain-boundary resistivity. The validity of this technique was illustrated using the examples such as the design of a new scavenger material and an analysis of the diffusion of the siliceous phase along the grain boundary.

A new approach to improve the grain-boundary conduction in 10 mol% gadolinia-doped ceria (GDC) without any additives was suggested. The grain-boundary conductivity of GDC specimen containing 500 ppm of SiO 2 was increased ∼4 times by post-sintering heat-treatment (HT) at 1350 °C for 20 h. The grain-boundary conductivity showed the maximum at HT temperature of 1350 °C and enhanced with increasing HT time from 0 to 20 h. The mechanism for scavenging resistive siliceous phase by post-sintering HT was investigated with the variation of HT temperatures, HT times, and HT schedules.

Impedance spectroscopy with spatial resolution using a microcontact array has successfully been applied to nitrogen-graded zirconia to establish the conductivity variation with nitrogen dopant concentration. Impedance profiles were measured in two configurations: 1) using one microelectrode versus a large counter electrode and 2) measuring between two microelectrodes. A versatile equivalent circuit model was introduced which allows a straightforward analysis of the obtained spectra which exhibit inductive loops as frequently associated with two-microcontact measurements. The analysis unambiguously revealed a local conductivity anisotropy in the material.

The grain-boundary resistivity of CaO·2SiO 2 -in-diffused 8 mol%-yttria-stabilized zirconia (8YSZ-CS) or SiO 2 -in-diffused 8 mol%-yttria-stabilized zirconia (8YSZ-S) was determined by local impedance spectroscopy using submillimeter-scale electrodes. During sintering, a liquid formed at the top of the 8YSZ penetrates or diffuses into the 8YSZ interior. For CaO·2SiO 2 -in-diffused 8YSZ, the grain-boundary resistivity of the specimen surface was observed to be 150 times greater than that of the interior, and grain growth was enhanced near the surface region. In contrast, for SiO 2 -in-diffused 8YSZ, the near-top surface region did not show enhanced grain growth and its grain-boundary resistivity was only nine times higher than that of the specimen interior.

When sintered 2Y-TZP (2 m/o yttria doped tetragonal zirconia polycrystals) specimens were embedded in ZrN powder and heat-treated at 1700°C, the cubic phase was stabilized as a result of nitrogen incorporation. The resultant microstructure was similar to a columnar zone in a metal alloy ingot: elongated grains were developed with their length parallel to the nitrogen flux into the specimen. Impedance measurements were made on the nitrogen-stabilized zirconia along the length direction of the columnar grains and perpendicular to the length direction, respectively. The boundary effects of the columnar grains were thus unequivocally distinguished. Different activation energies for the grain boundary and bulk conductivity support the blocking nature of the boundaries in nitrogen-stabilized zirconia as well as in 2Y-TZP. SEM and TEM analyses revealed that the nitrogen-stabilized zirconia consists of a matrix of a cubic phase containing a dispersion of tetragonal precipitates similar to the PSZ (partially stabilized zirconia). No continuous glassy phase at the grain boundaries was identified in the nitrogen-stabilized zirconia as well as in the 2Y-TZP ceramics. The grain boundary resistance was ascribed to a depletion space charge layer at the grain boundary.