Phase transition and electrical properties of Ni_1−x Zn _x Mn₂O₄ (0 ≤ x ≤ 1.0) NTC ceramics

The Ni_1−x Zn _x Mn₂O₄ (0 ≤ x ≤ 1.0) negative temperature coefficient (NTC) ceramics were prepared by the traditional solid state reaction method. X-ray diffraction results show that the as-sintered ceramics transformed to tetragonal spinel (x = 1) gradually from cubic spinel (x = 0). Scanning electron microscope images show that the shape of grains changed with increasing Zn ions content, because of the stress in grains induced by phase transition. The resistivity decreased slightly with increasing Zn ions content for x ≤ 0.25, but increased remarkably for higher Zn ions content. The values of ρ ₂₅, B _25/50 and activation energy for Ni_1−x Zn _x Mn₂O₄ (0 ≤ x ≤ 0.8) NTC thermistors were in the range of 3,511–136,780 Ω cm, 3,913–4,491 K and 0.3271–0.3691 eV, respectively. The values of relative resistance drift △R/R ₀ (0 ≤ x ≤ 0.8) were <2 % and had a minimum of 0.69 %, after aging test for 500 h. According to the complex impedance analysis, both of the NTC characteristic (0 ≤ x ≤ 1.0) and the increase of resistivity with adding Zn ions content (x > 0.25), were attributed to the increase of grain boundaries resistance.