Investigation of the electrical properties of SnO2 varistor system using impedance spectroscopy

The electrical properties of tin oxide varistors doped with CoO, ${\mathrm{Nb}}_{\mathrm{2}}{\mathrm{O}}_{\mathrm{5}},$ and ${\mathrm{Cr}}_{\mathrm{2}}{\mathrm{O}}_{\mathrm{3}},$ were investigated using the impedance spectroscopy technique with the temperature ranging from 25 to $\text{400\hspace{0.17em}°}\mathrm{C}.$ The impedance data, represented by means of Nyquist diagrams, show two time constants with different activation energies, one at low frequencies and the other at high frequencies. These activation energies were associated with the adsorption and reaction of ${\mathrm{O}}_{\mathrm{2}}$ species at the grain boundary interface. The Arrhenius plots show two slopes with a turnover at $\text{200\hspace{0.17em}°}\mathrm{C}$ for both the higher and lower frequency time constants. This behavior can be related with the decrease of minor charge carrier density. The barrier formation mechanism was associated with the presence of ${\mathrm{Cr}}_{\mathrm{Sn}}^{{\cdot }}$ at the surface, which promotes the adsorption of the ${\mathrm{O}}^{\prime }$ and ${\mathrm{O}}^{″}$ species which are in turn proposed as being responsible for the barrier formation.