  The charge transport mechanism and the macroscopic dielectric constant in
polycrystalline device materials commonly exhibit several components such as
electrode-sample interface, grain boundary and bulk contributions. In order to
gain precise understanding of the functionality of polycrystalline
electroceramic device materials it is essential to deconvolute these
contributions. The paradigm of functional thermistor ceramics based on thick
film spinel manganates has been studied by temperature dependent alternating
current impedance spectroscopy. Three typical relaxation phenomena were
detected, which all showed a separated temperature dependence of resistivity
consistent with thermally activated charge transport. The dominating grain
boundary and the interface contributions exhibited distinctively different
capacitance allowing clear identification. The composite nature of the
dielectric properties in polycrystalline functional ceramics was emphasized,
and impedance spectroscopy was shown to be a powerful tool to account for and
model such behaviour.
