Improvement of surge current performances of ZnO varistor ceramics via C3N4-doping

ZnO-Bi2O3 based varistor ceramics doped with C3N4 were fabricated via solid state method. Experimental results show that C3N4 additive acts as an inhibitor in ZnO grain growth, and the average grain size decreases from 10.2-7.1 mu m. The varistor breakdown voltage gradient increases from 222.3-282.3 V/mm, and the nonlinear coefficient increases from 51.9-58.2 with the increase of C3N4 content from 0 to 3.0 wt%. The C3N4-added samples exhibited smaller residual voltage ratio and better surge current withstanding capability. It is proposed that the C3N4-doping leads to substitution of nitrogen for oxygen in the grain boundary region, forming acceptor type defects. The acceptor type defects act as electron traps, increasing the barrier height from 1.31 to 1.50 eV and the depletion layer width from 54.9-61.8 nm, which increases the nonlinearity, and the surge current performances of the C3N4 doped ZnO varistor ceramics are improved.