Microstructure and varistor properties of Pr–Co co-doped ZnO ceramics obtained by sol–gel method

Zn_0.99−xCo_0.01Pr_xO (x = 0, 0.005, 0.015 and 0.020) nanoparticles for varistor application have been synthesized by sol–gel method. The phase and microstructure of the as-prepared samples were characterized by X-ray diffraction, transmission electron microscopy, scanning electron microscopy and energy dispersive X-ray spectra. The results reveal that the samples exhibited hexagonal zinc oxide (ZnO) lattice structure without secondary phases and were well crystallized with preferential orientation towards (101) direction. The calculated average crystalline size decreases from 39.9312 to 39.3605 nm when Pr³⁺ content increased from 0 to 2 at.%. The microstructure of Zn_0.99−xCo_0.01Pr_xO (x = 0.02) varistors ceramics consisted of ZnO grain as a primary phase and Pr-rich phase (Pr₆O₁₁) and spinel phase (Pr₂CoO₄) as minor secondary phase. The breakdown field increased over a wide range from 2332 to 7845 V/cm with the Pr³⁺ content increasing. The varistors doped with 1.5 at.% Pr³⁺ exhibited the highest nonlinear coefficient of 17.9. Further increase caused nonlinear coefficient decreased to 15.4 at 2 at.%. The current–voltage (E–J) characteristics curves indicated the Pr–Co co-doped ZnO varistors showed good electrical properties and could be a promising material to replace ZnO–Bi₂O₃ varistors which is appreciable for the fabrication of voltage switching devices in the near future.