Electrode-contact spreading resistance phenomena in doped-lanthanum gallate ceramics

Impedance spectra of La0.8Sr0.2Ga0.83Mg0.17O2.815 ceramics were measured using gold and platinum paste, and sputtered gold electrodes. Impedance complex plane plots, Z*, for samples with sputtered gold electrodes consist of a single arc associated predominantly with the bulk response and a low frequency 'spike' associated with electrode-polarisation and diffusion-related phenomena. An additional arc at intermediate frequencies with an associated capacitance of the order of 1 nFcm(-1) and a resistance with a similar temperature dependence to that of the bulk resistance is present in Z* plots for samples measured using gold paste electrodes hardened at temperatures between 500 and 850degreesC. Scanning electron microscopy and impedance spectroscopy show the additional response to be a 'spreading resistance' caused by the electrodes making well-separated discrete contacts with the ceramic due to partial sintering of gold particles in the paste. Z* plots for samples measured using platinum paste electrodes hardened between 725 and 1000degreesC exhibit a much smaller additional response with similar behaviour to that observed with gold paste electrodes but also showed an apparent increase in sample resistance due to significant loss of contact between sample and electrode in some regions of the interface.