Electrical Properties And Aging Phenomenon Of Si Doped Nicomncuo Negative Temperature Coefficent Themistors

Transition metal manganite oxides are widely used as Negative temperature coefficient themistors (NTC) in electronic circuits of industrial and household applications. NTC thermistors with lower resistance and good sensitivity can be used to suppress in-rush current. Copper-containing NTC material exhibits higher conductivity than copper-free ones, but lower sensitivity to the temperature. The introducing of Ni and Co is able to obtain copper containing NTC materials with high B constant, but raises the production cost. In this paper, the concentration of Ni and Co was both limited to 10mol%, SiO₂ was introduced into Ni_0.3Co_0.3Mn_2.4-xCu_xO₄ to prepare Copper-containing NTC materials with low reisistivity, good sensitivity and lower production cost.The electrical properties of transition metal manganite oxides strongly depend on the ionic state and the distribution of the cations in the tetrahedral site (A site) and octahedral site (B site) of the spinel structure. The cation disrtibution of Si-doped Ni_0.3Co_0.3Mn_2.04Cu_0.36O₄ and Ni_0.3Co_0.3Mn_2.28Cu_0.12O₄ was studied in Chapter III by XPS and XRD experimental methods. The results show that Cu ion presents as Cu⁺ and Cu²⁺ in tetrahedral sites when SiO₂ was introduced to Ni_0.3Co_0.3Mn_2.04Cu_0.36O₄. And the indirect jump between Mn³⁺ and Mn⁴⁺ on more distant B sites with the assistance of Cu²⁺ in A sites is responsible for the decrease in resistivity and the increase in B25/50 constant. In Si-doped Ni_0.3Co_0.3Mn_2.28Cu_0.12O₄, indirect jump does not take place without the existence of Cu_A²⁺.The effect of SiO₂ addition on Ni_0.3Co_0.3Mn_2.4-xCu_xO₄ was studied systematic in chapter IV. It is found that the appropriate mount of SiO₂ addition is desirable for the demands of themistors with low resistivity and high sensitivity when X>0.12.Aging phenomenon was studies by the ionic configuration of the samples before and afer aging test. The drift rate (â–³R/R) in Ni_0.3Co_0.3Mn_2.16Cu_0.24O₄ is -1.6%, and increases to 19.6% when Si was introduced, and finally reduce to 0.3% with Li and Si co-doping. The XPS results demonstrate that the cation redistribution is responsible for the drift of resistivity during aging. There are two factors which influence cation redistribution: (1) The unstable Cu+ ion is apt to change to Cu2+, causing lower Mn4+ concentration in octahedral sites. (2) The concentration of absorbed oxygen increases during aging, which results in more Mn ion with higher oxidation state. Factor (2) dominantly affectsâ–³R/R in Si-doped Ni_0.3Co_0.3Mn_2.16Cu_0.24O₄, causing high resistivity change during aging. Li doping reduceâ–³R/R by derease the effect of factor (2) , and no harmful changes of resistivity and B constant were observed in Li doped samples. These demonstrate Li and Si co-doping is desirable for the fabrication of stable themistors with low resistivity and high sensitivity.