Preparation And Properties Study Of Nickel Manganese High Density Nano-negative Temperature Coefficient(NTC) Materials

The negative temperature coefficient(NTC)thermistor,one of temperature sensitive materials,was widely used in household appliances,manufacturing,healthcare,and automotive industries field.As submicron-,nano-and micro-electronic systems are developed vigorously,the materialization of nano ceramic devices in electronics has become an inevitable trend.In this study,Ni-Mn-O and Ni-Mn-Fe-O systems,two spinel materials,were prepared by sol-gel method,and thermo-sensitive nano-blocks were fabricated via two-step sintering approach.The effects of preparation process and raw material ratio on the microstructure and electrical properties of fabricated ceramics were studied.This study provides some implications for the application of NTC thermo-sensitive materials of nano-particles.This thesis is structured in the following way:First,Ni-Mn-O NTC thermistors with high electrical stability were fabricated by a novel two-step sintering approach.Nickel-manganese spinel powder,with an average grain size of 50 nm,was used as a raw material.A novel two-step sintering approach that integrates rapid sintering process with conventional two-step sintering approach was used to obtain NTC thermistors.The average grain sizes of sintered thermistors range from 254 nm to 570 nm,and the relative density from 84%to 91%.The minimum aging coefficient of thermistors which were aged at 150? for 500 hours is only 0.1%,indicating that the cation or cation vacancies tend to be thermodynamically balanced.Therefore,this method provides a way to obtain high electrical stability of NTC thermistor.In addition,to obtain thermistors with higher temperature sensitivity(higher B value),the author investigated the effect of doping on the electrical properties of Ni0.54Mn1.26Fe1.2-xZnxO4(0<x<0.6).The results show that the B50/80 values decrease with the increase of Zn content,which is because the increase of Zn content leads to the increase of the hopping conductive ion pairs(Mn3+/Mn4+)in the octagonal sites,a phenomena that results in the increase of electrical resistance.In general,the greater the resistance or resistivity,the greater the B value.The B50/80 value of Ni0.54Mn1.26Fe1.2O4 thermistors obtained in the present work is 5432 K,the highest value among all thermistors of Ni-Mn-Fe-O system,which means that Ni0.54Mn1.26Fe1.2O4 is more sensitive to temperature than others.Finally,Ni0.54Mn1.26Fe1.2O4 NTC thermistors with the highest temperature resistively,according to the result of the second part,were fabricated by the conventional two-step sintering approach.The effect of sintering process on the microstructure and electrical properties of nano NTC thermistors was investigated.According to the microstructure study of fabricated thermistors,increasing the temperature of the first sintering step(1000-1180?)significantly reduces the pores and results in clearer grain boundaries.The relative density and grain size of thermistors are increased by increasing the temperature of the second sintering step(1000-1080?).The electrical properties results show that all B values are greater than 5000 K,the aging coefficient is 0.85%?5.51%,and the aging coefficient is decreased with the increase of density because it is much easier for conductive ions in dense material to reach electrical stability during the second sintering stage.It is demostrated by complex impedance data that grain resistance and grain boundary resistance show typical NTC characteristic and the latter is larger than the former.This indicates that grain boundaries play a dominant role in resistance of nanostructured NTC ceramics.