The effect of lithium oxide on the threshold field of ZnO varistors

Lithium oxide in form of Li2CO3 solution is added with contents of 0–200 ppm to two ZnO-based varistors standard formulations, once with Sb2O3 and the other without. According to Li2CO3 concentration, both threshold field and energy absorption capability evolution are studied. It is found that with the benefit of antimony, the lithium allows reaching high threshold field but concomitantly, low energy absorption capability. Without antimony, threshold fields up to 300 V/mm are attained, associated with a fair energy absorption capability. With 100 ppm of Li2CO3, optimum couple of values (315 V/mm; 115 J/cm3) is achieved. With 200 ppm of Li2CO3, threshold field exceeds 500 V/mm but energy absorption capability falls below 50 J/cm3. Correlations with SEM microstructures observations suggest that lithium increases voltage barrier height by decreasing donor density and that spinel phases (Zn7Sb2O12) have detrimental effects on the electrical absorption capability by limiting the density of current, reducing the effective current path from one ZnO grain to another.