Preparation And Properties Study Of Core-Shell NTC Thermistor Materials

Negative temperature coefficient(NTC)thermistors can be used for temperature measurement,temperature control,compensation and surge suppression,driven by its resistance-temperature relationship where the resistance decreases with the growing temperature and its voltage-current characteristic.The most important parameters for NTC materials are material constant(B),electrical resistivity(p)and aging rate,all of which can be regulated by the chemical composition and/or microstructure of the material.In this paper where the Ni-Mn spinel material was focused,the material's electrical properties were regulated from the perspective of core-shell structure,and the impact of core/shell ratio on the electrical properties was highlighted.The specific work done was described as below.In the first part,Ni0.7Mn2.3O4 spinel powder with an average particle size of about 50 nm and Al2O3 sol were prepared by the sol-gel method,and Ni0.7Mn2.3O4@Al2O3 core-shell powder was obtained.The core-shell power was sintered at 1250? to produce Ni0.7Mn2.3O4@Al2O3 thermistors.The influence of Al2O3 ratio(10 at%,20 at%,30 at%,40 at%,50 at%)on the electrical properties of the thermistors material was studied.The results showed that the B value of the materials ranged from 3868 K to 5878 K and the resistivity from 3646 ?·cm to 69 M?·cm;both parameters increase as the Al2O3 ratio gets higher.The grain boundary resistance of Ni0.7Mn2.3O4@Al2O3 is obviously greater than the grain resistance and acts as the dominating resistance in the material,and both resistances show NTC characteristic.In the second part,the thermal sensitive Al2O3@Nio.7Mn2.3O4 material was prepared through the above-described procedures.Then,the influence of Ni0.7Mn2.3O4 content(10 at%,15 at%,20 at%,25 at%,30 at%)on the electrical properties of the material was studied.According to necessary measurements and analyses,B value of the material ranges from 3967 K to 4556 K and the resistivity from 17979?·cm to 91919 ?·cm.Both parameters increase first and then decrease as the Ni0.7Mn2.3O4 content increases,because the content increase results in more Mn3+/Mn4+pairs,which in turn lowers the resistivity and B value;however,when the content reaches to such an extent that grains become smaller and grain boundaries are increased,leading to higher grain boundary potential barrier and higher B value.Both the grain resistance and grain boundary resistance decrease with the increase of temperature,but the former is always greater than the latter,indicating that the grain resistance plays a dominant role.In the third part,Ni0.7Mn2.304@Al2O3-30 at%and Al2O3@Ni0.7Mn2.3O4-30 at%core-shell structured materials were prepared by the SPS technique and the effect of sintering temperature on the properties of the core-shell structured materials was studied.According to necessary measurements and analyses,as the sintering temperature increases,the materials become denser and the B values first get larger and then smaller.Both the grain boundary resistance and grain resistance in the Ni0.7Mn2.304@Al2O3-30 at%material show NTC characteristic,and the latter is greater than the former,indicating that the grain resistance plays a dominating role.On one hand,the grains of SPS sintered material are significantly smaller than those of the solid phase sintered counterpart,on the other hand,only grain boundary resistance exists in Al2O3@Ni0.7Mn2.3O4-30 at%material and shows the NTC characteristic,this suggests that the grain boundary resistance plays a dominant role.