Investigation On Mn-Ni-Cu-O-based NTC Thermistors With Low ρ And High B Value

Negative Temperature Coefficient Resistor are widely used for suppression of inrush current, temperature measurement,temperature compensation of circuits and so on.To obtain a NTC thermistor material with low resistivity and high material constant B value, the Mn-Ni-Cu-O system was chosen to study the effects of chemical composition and preparation process on the phase composition, microstructure and electrical properties.The influences of chemical composition and sintering temperature on the microstructure and electrical properties of the Mn-Ni-Cu ternary system were studied. The results show that when the Ni content is within the range of 0.35≤x≤0.65,the material constant Brises along with theincreasing Ni content, but the the room temperature resistivity firstly increases and then decreases with the increasing Ni content.The complex impedance spectroscopy analysis shows that the variation of nickel content will cause the cationsto redistribute in grain, which leads to a corresponding change in resistivity. And XRD analysis shows that excessive Ni contentis responsible for the formation of undesirable phase NiO, which will also result in an increase of resistivity. With the increase of Cu content, theroom temperature resistivityand material constant B first decline and then increase, the sintering temperature is very sensitive to the Cu content, the optimal sintering temperature declineswith the increasing Cu content,and excessive sintering temperature wouldcause theroom temperature resistivity to increase.Based on the experimental results of Mn-Ni-Cu ternary system, the effects of Fe2O3 addition on microstructure and electrical properties of Mn-Ni-Cusystem were investigated. XRD analysis shows that Fe2O3 doping does not change the cubic spinel structure, both the lattice constant and the grain size of as-sintered ceramics decrease with the Fe content increases. The resistivity of the samples firstly decrease slightly with Fe content x from 0 to 0.1, and then, the resistivity increases abruptly at the Fe content x≥0.2. On the other hand, the B value firstly decreases and then increases, and obtains a minimum value at x=0.3The influences of SiO2 doping on the properties of Mn-Ni-Cu ternary system were investigated. When the Si content is less than 0.08,the grain size decreases and the number of grain boundaries increases with the Si content increases. And it behaves conversely when the Si content is higher than 0.08. Si ions presents as Si4+ mainly occupy the octahedral sites(B site), the distribution of the cations in the spinel structure varies with Si content, resulting in the change of the electrical properties of the samples.The effects of Co doping on the microstructure and electrical properties of Mn-Ni-Cusystem were investigated. The results show that all Co ions enter into the lattice at the Co content x<0.8, but At x=0.8, Co3O4 and CoO phases can be observed besides the main cubic spinel phase.The porosity reduces and the bulk density increases with increasing Co content. The electrical properties exhibit a significant increase in B value while the resistivity changes little. Under the influence of Co, the resistivity of the material increases slightly, while the B value increases significantly.The effects of Zn addition on the properties of Mn-Ni-Cusystemwere studied. The materials exhibit single cubic spinel structure at the Zn content x<0.7, but it turns into tetrahedrally distorted spinel structure at x=0.7, and there also exist impure phases NiO and CuO. Both the resistivity and B value increase with the Zn content increases, and the aging properties improve with the increasing Zn content, and the resistance drift / obtain the minimum value 1.47% after aging test at the Zn content x=0.6.