Structural morphology and nonlinear behavior of pure and co-doped Zn_1-x-yFe_xM_yO varistors with (M = Cu, Ni)

We report here structural morphology and nonlinear behavior of pure and co-doped Zn_0.90-xFe_0.1M_xO with (M = Cu, Ni and (x = 0.00, 0.10) and (0.00 ≤ y ≤ 0.20)) at different sintering temperatures (T _s = 850 and 1000 °C). It is found that the co-doping of ZnO by (Fe + Cu) or (Fe + Ni) up to 0.30 does not deform the well-known wurtzite structure of ZnO, as well as pure and 0.1 of Fe-doped ZnO. The SEM micrographs did not show any secondary phases at the boundaries of grains as compared to ZnO, the average grain size is decreased for Fe and (Fe + Cu) samples, while it is increased for (Fe + Ni) samples. The nonlinear coefficient α and breakdown field E _B are generally increased by 0.1 of Fe addition, but they are shifted to lower values as T _s increases for all samples. Furthermore, they are gradually increased/decreased to higher/lower values for (Fe + Cu/Fe + Ni) samples up to 0.30 of co-doping content. The values of α and E _B are increased from 30.06, 2115.38 V/cm for ZnO at 850 °C to 50.07, 5012 V/cm by (0.1Fe + 0.2Cu) co-doping, and from 23.53, 1956.52 V/cm to 45.58, 4750 V/cm at 1000 °C, while they are, respectively, decreased by (0.1Fe + 0.2Ni) to 13.19, 312 V/cm and 11.85, 172.42 V/cm. Similar behavior was generally obtained for nonlinear conductivity σ _L and height of potential barrier φ_B, whereas the vice is versa for the behavior of leakage current J _k and residual voltage K _r. Our results are discussed in terms of the comparative participation between the effects of co-doping of (Fe + Cu) and (Fe + Ni) to ZnO for supporting the potential barrier as compared to individual doping by Fe, Cu and Ni. This study perhaps recommended these samples for optoelectronic and ferromagnetic investigation after COVID-19 is over.