Preparation and Characterization of NTC Thermistors: Nickel Manganites Doped with Lithium

Oxides of the form Ni x Mn 3‐x O 4 present properties that make them useful as NTC thermistors. Nickel manganites doped with lithium were synthesized by two methods: insertion with n ‐BuLi and reaction of nickel‐manganese formates with lithium formate. XPS and XRD studies showed that both procedures led to oxides with spinel structure, although with different cation distributions. Dilatometry established that a cationic reordering took place on manganites doped by insertion with n ‐BuLi during sintering. SEM mapping studies showed that a segregation on grain boundaries took place, leading to an increase in Mn content and a decrease in nickel on the particle surface. Finally, electrical studies showed the nickel manganites doped with lithium, Ni x Mn 3‐x‐y Li y O 4 , had similar resistivities and sensitivity indexes to nickel manganites, Ni x Mn 3‐x O 4 , but higher electrical stability.