Preparation And Electrical Properties Of High Performance NTC Thermistor

In this paper,negative temperature coefficient（NTC）thermistors,including spinel system,perovskite system and composite system,were fabricated by a solid-state reaction.For the Mn-Ni-Cu system,the effect of Cu content on the microstructures and electrical properties was investigated.In order to improve the thermal stability,two different dopants of ZnO,Cr₂O₃ were introduced into the Mn-Ni-Cu system.For the LaMnO₃ system,their resistivity and B value was adjusted and improved by doping TiO₂、Al₂O₃ and Cr₂O₃.Furthermore,the crystal structures,morphologies and electrical properties of the composite ceramics mixed by Ni_0.75Mn_2.25O₄ and LaMnO₃ with different proportions were characterized.The electrical stability of composite ceramics was investigated.1.For the Ni-Mn-Cu based NTC thermistor,the crystal structures of Ni_0.7Mn_2.3-x Cu_xO₄（0≤x≤0.4）samples have cubic spinel structure.With the increase of Cu content,the material constant B decreased from 3929.2K to 2019.2K,and the room temperature resistivityρ₂₅decreased from 780.9Ω·cm to 15.7Ω·cm.However,the thermal stability was worse.The effect of ZnO on the electrical properties and the stability of Ni_0.7Mn_2.0-xCu_0.3Zn_xO₄（x=0⁰.7）NTC thermistors was investigated.The results show that ZnO has little effect on the room temperature resistivityρ₂₅ and the material constant B.However,the thermal stability has been significantly improved,leading to the minimum drift value of 1.52%in resistivity when the ZnO content is x=0.7.The effect of Cr₂O₃ dopant on the electrical properties and the stability of Ni_0.7Mn_2.0-xCu_0.3Cr_xO₄（x=0⁰.4）NTC thermistors was investigated.The addition of Cr₂O₃ do not cause much change in the electrical properties and thermal stability.2.For the LaMnO₃ based NTC thermistor,the lattice position of Mn ions was substituted by TiO₂,Al₂O₃ and Cr₂O₃ to increase the room temperature resistivityρ₂₅ and the material constant B of the samples.Both theρ₂₅ and B value of the LaMn_1-xTi_xO₃（x=0⁰.4）NTC thermistors increased with the increase of Ti amount.The values ofρ₂₅ and B constant of the NTC thermistors with different compositions are 2.83Ω·cm¹390.35Ω·cm,650.496K²165.95K,respectively.The addition of Cr and Al did not cause much change in the electrical properties.3.The electrical properties of(Ni_0.75Mn_2.25O₄)_1-x（LaMnO₃）_x（x=0.1⁰.6）system were studied to obtain electrical performance adjustable NTC thermistor with a wide range of temperature.Theρ₂₅ and B value of the composite ceramics decreases with increasing x.The values ofρ₂₅ and B constant of the NTC thermistors with different compositions are15705.44⁵7.61Ω·cm,4431.51¹661.83K,respectively.This means that the electrical properties can be adjusted to desired values,depending on their perovskite phase content.Moreover,the introduction of the perovskite phase led to a decrease in the resistance drift.