Preparation Of Spinel Phase Of Fe-mn-ni-o Negative Temperature Coefficient Thermistors And Aging Properties

Negative Temperature Coefficient (NTC) ceramic are increasingly used in various industrial and domestic applications, such as elements for the suppression of in-rush current, for temperature measurement and control, and for the compensation of other circuit elements. The specific resistivity of these ceramics follows the well known Arrhenius relation: ρ=ρ0exp(E_α/kT), in which ρ is the specific resistivity, E_α is the activation energy for electronic conduction, k is Boltzmann's constant and T is absolute temperature. The thermal coefficient is defined as α=-B/T², with B=E_α/k. These ceramics often consist of 3d transition metal oxides with spinel structure, such as Mn-Ni-O, Mn-Ni-Co-O, Mn-Ni-Fe-O and Mn-Ni-Cu-O etc. Electrical parameters of these ceramics including electrical resistivity and B value are determined by the distribution of cation in the spinel, and these parameters are also affected by the microstructure. In other, a drift in resistivity of the NTC ceramics with time is termed aging. Some prevailing mechanisms include the redistribution of cations between A-site and B-site in spinel and the migration of cation vacancies from grain boundary to bulk. This dissertation is to investigate the electrical properties and aging behavior in Mn-Ni-Fe-O systems, and correlate these properties with distribution of cations in the spinel, and to explore new approaches to suppress the aging of the NTC ceramics.Chapter 1 gives a brief introduction to the history, basic parameters, classification and applications of NTC ceramics and thermistors. The studies on crystal structure, cation distribution, electronic conduction, and aging mechanism of spinel-type NTC ceramics as well as their preparation methods are also reviewed. Finally, the scope of this dissertation is described.In Chapter 2, the Pechini method has been adopted to prepare FeMnNiO₄ powder. It is shown that the primary particle size of the powder can be controlled by adjusting the ethyl glycol content. The compact of the powder with small primary particle size can be sintered to dense at 1050℃. The resulting ceramic possesses small grain size...