Dependence of non-linearity coefficients on transition metal oxide concentration in simplified compositions of $ZnO+Bi_20_3+MO$ varistor ceramics (M=Co or Mn)

Abstract

Ceramics with simplified compositions of $ZnO + Bi_20_3 + CoO \hspace{2mm} or \hspace{2mm} MnO$ show non-linearity coefficients $(\alpha)$ of 40-65 provided the concentration of transition metal ions is > 1.5mo1%. Samples doped with Co have higher non-linearity coefficients than those with Mn. This is attributed to the stability of multiple oxidation states of Co(Ill) + Co(ll) in contrast to Mn(ll) as the stable species in sintered ZnO ceramics. Electron paramagnetic resonance and diffuse reflectance spectral studies establish this fact. Low-signal capacitance-voltage measurements show that donor density $(N_d)$ in these ceramics ranges from 0.3 to $1.8 \times 10^{19} cm^{-3}$, which is comparatively larger than those of commercial varistors. The barrier height $(\phi_b)$ reaches up to 0.66 eV and the breakdown voltage is around 3.4-3.7 eV. Admittance spectroscopy and isothermal capacitance transient spectroscopy (ICTS) are used for characterizing the bulk traps originating from the transition metal dopants. Capacitance-voltage analyses above the breakdown voltages show negative capacitance, indicative of oscillatory charge redistribution involving multivalent states of Co and the shallower interface states. Multiple trapping relaxations are evident from the complex-plane capacitance studies