Aging of nickel manganite NTC ceramics

Effect of thermal history and chemical composition on aging of Ni _x Mn_3 − x O_{4 +  δ} (0.56 ≤ x ≤ 1.0) ceramics was investigated. It was found that all the Ni _x Mn_3 − x O_{4 +  δ} ceramic samples metallized by co-firing at 1050°C showed significant electrical stability with an aging coefficient less than 1.0%, while aging of those metallized by annealing at 850°C was increasingly serious with a rise in Ni content x, the aging coefficient ranging from 0.2% to 3.8%. However, the ceramic samples with Ni content x ≤ 0.70, whether metallized by co-firing or by annealing, exhibited extraordinarily high electrical stability with an aging coefficient less than 0.5%. The composition dependence of aging of the ceramic samples was explained qualitatively, based on the electrical conduction mechanism of small polaron hopping and on the aging mechanism of the cationic vacancy-assisted migration of cations to their thermodynamically preferable sites under thermal stress.